An Adventure: Weightlessness Phenomena and Life on Space Station 9811992207, 9789811992209

Table of contents :
Preface: Free Fall and Weightlessness in Space Travel
Contents
1 Introduction: Why Do We Fly into the Space?
2 Free Fall and Weightlessness in Gravity
2.1 The Story of Isaac Newton and the Apple Falling
2.2 Why Does a Cannonball Fall Down to Earth but the Moon Does Not?
2.3 Why Do Scientists Launch Spacecrafts from the Equator?
2.4 Weight and Weightlessness
2.5 The Wonderful World of Microgravity
3 Measure the Body Weight on Space Station
3.1 Measure the Weight on the Ground
3.2 Measure the “Weight” or Mass of Astronaut on Space Station
3.3 Why Do We Confuse Weight and Mass?
3.4 Mass and Weight from North Pole to Equator
3.5 How is Earth’s Mass Measured?
3.6 Short Summary on Mass and Weight
4 The Space Swing Never Stops
4.1 Constructing a Simple Pendulum
4.2 A Pendulum Anchored in the Cosmos
4.3 The Mystery of Swinging
4.4 Keeping Time with a Pendulum
4.5 Simple Pendulum Motion in Gymnastics
4.6 A Pendulum in Weightlessness State
5 The Gyroscope on Space Station
5.1 Stability of a Spinning Gyroscope
5.2 Gyroscope in Weightlessness
5.3 Gyroscopes in Our Daily Life
5.4 Gyroscopes in Universe: Earth and Pulsar
6 Wonders of Water in Weightlessness
6.1 Shape of Water Drop in Weightlessness
6.2 Thin Film of Water on a Space Station
6.3 Super Water Globe Ball and Cocktail on a Space Station
6.4 Amazing Surface Tension
6.5 Disappearance of Buoyancy Force in Weightlessness
7 Viewing the Universe and Earth on a Space Station
7.1 Why are Stars not Twinkling Seen from the Space Station?
7.2 Space Station: 16 Times Sunrise or Sunset in One day
7.3 Why is Earth Shown as a Blue Planet?
7.4 Moon is Much Brighter Seen from the Space Station
7.5 Can the Naked Eye See the Great Wall from the Space Station?
7.6 Space Telescope from Hubble to James Webb
8 Wonderful Space Life: Part One
8.1 Playing with Fire on the Space Station: Round Flame
8.2 Will People Shed Tears on the Space Station?
8.3 Will the Astronauts Become Taller in Space?
8.4 Spacewalking Outside of a Spacecraft
9 Wonderful Space Life: Part Two
9.1 Sleeping in Weightlessness
9.2 Drinking and Eating on a Space Station
9.3 Brushing Teeth and Taking Shower on a Space Station
9.4 Space Toilet and Water Cycling System
10 Extended Knowledge and Products Based on Space Exploration
10.1 Effects of Weightlessness on Women and Men
10.2 Animals Respond to Weightlessness
10.3 Risk of Cosmic Rays and Solar Wind Storm on Astronauts
10.4 Plant Growth on Space Station
10.5 Broad Knowledge Inside the Spacesuit
11 Postscript: Flying into Space is a Human Dream

Citation preview

Chengmin Zhang

An Adventure Weightlessness Phenomena and Life on Space Station

An Adventure

Chengmin Zhang

An Adventure Weightlessness Phenomena and Life on Space Station

Chengmin Zhang National Astronomical Observatories, University of Chinese Academy of Sciences, Chinese Academy of Sciences Beijing, China

ISBN 978-981-19-9220-9 ISBN 978-981-19-9221-6 (eBook) https://doi.org/10.1007/978-981-19-9221-6 © Springer Nature Singapore Pte Ltd. 2023 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Preface: Free Fall and Weightlessness in Space Travel

With the advent of the space age, global space travel is not the preserve of science fiction and dreams; it will happen in the Space Shuttle, from the South Pole to the North Pole, through Mount Everest, to the moon, and even to Mars. People are eager to understand the strangeness of weightlessness, the wonders of space, and the physics behind it. This book describes the physical principles of the special phenomena of weightlessness and other unusual experiences by astronauts on space station, and is of particular interest to public and the young students who want to explore and understand the space life of an astronaut. It features cartoons illustrating the extraordinary different scenarios between the space and Earth, analyzing the similarity in both cases as a comparison. From this book, readers can imagine the adventure of a space trip, and learn about the strange feelings of a weightless space station, as well as the physical causes behind these phenomena. The book will emphasize on the factual, scientific, and physics-based explanations for the phenomena that are discussed, presented using the easily understanding language and examples. Our purpose is to uncover the secrets of these extraordinary events in weightlessness, and inspire the readers to think and imagine the principles of gravity and physics. The author is grateful for the generous helps from the editors and coordinators of SpringerNature, e.g., Lillian Zhang, Swetha Divakar, Revathy Manikandan, and Jian Li, etc. Thanks are also due to the assistance from Stefanie Wachter, Donald W. Hoard and Yawen Liu for language suggestions, and author also expresses his gratitude to the painters, e.g., Shanshan Liu, Chen Liu, Fengjiao Liu, Jianye Jia, and Ni X., in making lots of efforts in the figure plots. Beijing, China

Chengmin Zhang

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Contents

1

Introduction: Why Do We Fly into the Space? . . . . . . . . . . . . . . . . . . . .

1

2

Free Fall and Weightlessness in Gravity . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1 The Story of Isaac Newton and the Apple Falling . . . . . . . . . . . . . . 5 2.2 Why Does a Cannonball Fall Down to Earth but the Moon Does Not? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.3 Why Do Scientists Launch Spacecrafts from the Equator? . . . . . . . 7 2.4 Weight and Weightlessness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.5 The Wonderful World of Microgravity . . . . . . . . . . . . . . . . . . . . . . . . 10

3

Measure the Body Weight on Space Station . . . . . . . . . . . . . . . . . . . . . . . 3.1 Measure the Weight on the Ground . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Measure the “Weight” or Mass of Astronaut on Space Station . . . . 3.3 Why Do We Confuse Weight and Mass? . . . . . . . . . . . . . . . . . . . . . . 3.4 Mass and Weight from North Pole to Equator . . . . . . . . . . . . . . . . . 3.5 How is Earth’s Mass Measured? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6 Short Summary on Mass and Weight . . . . . . . . . . . . . . . . . . . . . . . . .

11 11 12 13 14 16 16

4

The Space Swing Never Stops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Constructing a Simple Pendulum . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 A Pendulum Anchored in the Cosmos . . . . . . . . . . . . . . . . . . . . . . . . 4.3 The Mystery of Swinging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Keeping Time with a Pendulum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5 Simple Pendulum Motion in Gymnastics . . . . . . . . . . . . . . . . . . . . . 4.6 A Pendulum in Weightlessness State . . . . . . . . . . . . . . . . . . . . . . . . .

19 19 20 21 23 24 25

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The Gyroscope on Space Station . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Stability of a Spinning Gyroscope . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Gyroscope in Weightlessness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Gyroscopes in Our Daily Life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4 Gyroscopes in Universe: Earth and Pulsar . . . . . . . . . . . . . . . . . . . . .

27 27 30 30 34

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Contents

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Wonders of Water in Weightlessness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Shape of Water Drop in Weightlessness . . . . . . . . . . . . . . . . . . . . . . . 6.2 Thin Film of Water on a Space Station . . . . . . . . . . . . . . . . . . . . . . . 6.3 Super Water Globe Ball and Cocktail on a Space Station . . . . . . . . 6.4 Amazing Surface Tension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5 Disappearance of Buoyancy Force in Weightlessness . . . . . . . . . . .

37 37 39 39 43 49

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Viewing the Universe and Earth on a Space Station . . . . . . . . . . . . . . . . 7.1 Why are Stars not Twinkling Seen from the Space Station? . . . . . . 7.2 Space Station: 16 Times Sunrise or Sunset in One day . . . . . . . . . . 7.3 Why is Earth Shown as a Blue Planet? . . . . . . . . . . . . . . . . . . . . . . . . 7.4 Moon is Much Brighter Seen from the Space Station . . . . . . . . . . . 7.5 Can the Naked Eye See the Great Wall from the Space Station? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.6 Space Telescope from Hubble to James Webb . . . . . . . . . . . . . . . . .

51 51 53 54 57

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Wonderful Space Life: Part One . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1 Playing with Fire on the Space Station: Round Flame . . . . . . . . . . . 8.2 Will People Shed Tears on the Space Station? . . . . . . . . . . . . . . . . . 8.3 Will the Astronauts Become Taller in Space? . . . . . . . . . . . . . . . . . . 8.4 Spacewalking Outside of a Spacecraft . . . . . . . . . . . . . . . . . . . . . . . .

63 63 66 67 67

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Wonderful Space Life: Part Two . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1 Sleeping in Weightlessness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2 Drinking and Eating on a Space Station . . . . . . . . . . . . . . . . . . . . . . . 9.3 Brushing Teeth and Taking Shower on a Space Station . . . . . . . . . . 9.4 Space Toilet and Water Cycling System . . . . . . . . . . . . . . . . . . . . . .

71 71 74 76 77

10 Extended Knowledge and Products Based on Space Exploration . . . . 10.1 Effects of Weightlessness on Women and Men . . . . . . . . . . . . . . . . . 10.2 Animals Respond to Weightlessness . . . . . . . . . . . . . . . . . . . . . . . . . 10.3 Risk of Cosmic Rays and Solar Wind Storm on Astronauts . . . . . . 10.4 Plant Growth on Space Station . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.5 Broad Knowledge Inside the Spacesuit . . . . . . . . . . . . . . . . . . . . . . .

79 79 80 83 85 85

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11 Postscript: Flying into Space is a Human Dream . . . . . . . . . . . . . . . . . . 89

Chapter 1

Introduction: Why Do We Fly into the Space?

In the twenty-first century, more and more children curiously ask why humans have desired to fly into space?

I attempt to answer this question by presenting the best reason or excuse, however, after pondering for a while, I think that the best answer to a question might be not the best reason for it, thus I have to concentrate myself onto several good excuses that would not destroy the inborn curiosity of children. Taking myself as a civilized bird or animal, I would avoid the political contexts of the space exploration, although the United States, Europe Union, Russia, and China have shown a little bit different tendencies on the space station collaborations. To start, the earth is the cradle of lives, and it is impossible for mankind to lie in the cradle forever. It is our dream to fly into the space and look for a new planet for home. With the development of modern high-tech, especially aerospace and electronic information technology, our dreams have been planted with wings and traveling into the space can be expected soon (Fig. 1.1). The sun’s life span is about 10 billion years, and now it has been around 4.6 billion years old, an half of the total lifetime. As time goes by, the aging sun will keep expanding slowly, and eventually becomes a giant and red star. By then, the edge of the sun will be approaching our earth step by step. Its high temperature of several thousands of degrees will burn our earth and high-energy particle flows will impact and devour the earth, which is the inevitable end of the earth in the future 5 billion years. It may sound like still a long time before it comes, but if one day our earth suffers this serious impact by the asteroids accidently, then there is no doubt that we will die out like that the dinosaurs experienced about 60 millions of years ago. Therefore, it is going to happen that the human beings would find a new plant for our home to continue our lives in the future. Also, with the deterioration and overdraft of the earth’s ecological environment, the risk of human survival is increasing. A series of nightmares also urge the people to © Springer Nature Singapore Pte Ltd. 2023 C. Zhang, An Adventure, https://doi.org/10.1007/978-981-19-9221-6_1

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1 Introduction: Why Do We Fly into the Space?

Fig. 1.1 A parrot says to Isaac Newton, “Isaac, why are you lying in the cradle like a baby? Have you ever seen Albert Einstein? He’s flying into space”

explore the space and look for another planet, such as the expansion of uncontrolled population, global warming phenomenon, COVID-19 virus pandemic, limited earth resources, and so on. If human does not protect our environment and continue to cut down the forests, the Brazilian Amazon rainforest may disappear in the next 20 years. At that time, oxygen in the atmosphere will be reduced by one-fifth. Human and animals will confront the difficulties on their breaths. Therefore, the danger of human living environment is coming soon (Fig. 1.2). If the emission of carbon gets out of control and results in the greenhouse effect, then the deterioration of the climate will lead to the serious consequences. For example, the Arctic creatures, such as polar bears and seals, suffer threatened; a large quantities of fresh water stored in the Arctic glaciers will flood into the ocean and raise the sea level by 7 m; the coastal cities will also be affected, and the island of Maldives may be sunk into the Indian Ocean forever. If the Antarctic glaciers melted and the methane were released on a massive scale, a global warming would accelerate rapidly and the seriousness of consequences were unimaginable. Therefore, human survival on the earth is indeed facing a huge risk of global warming.

1 Introduction: Why Do We Fly into the Space?

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Fig. 1.2 Illustration of the earth flooding by the rise of sea level caused by the global warming that melts the Antarctica’s glaciers

At present, in the detected universe, the earth is the only planet with life. She has lived for 4.6 billion years from a lonely inorganic world to a carrier which is full of diverse lives. The earth is lucky enough to have the sun to provide the light and heat, the water to nurture all, the atmosphere to protect lives and the soil to preserve lives and nutrients…. The space gave the earth its peculiar blessing, created a rich species of world, formed more than two million food chains, and nurtured the highest intelligent creatures—humans. The Earth is the cradle of life and the solely homeland of human (Fig. 1.3). Is the earth really the only one planet which fosters the lives in the star ocean? There are about 200 billion galaxies in the universe, and about 200 billion solar systems in our galaxy. So it is not difficult to imagine that the universe will have many planets like the earth. Time waits for no man. Humans must be able to move to other planets and live on, or we will confront the same fates as dinosaurs did. Flying into the space, flying out of the solar system, and looking for alien planets are bound to be a hot topic in the twenty-first century. At present, over 5,000 exoplanets are confirmed to exist beyond our solar system by the recent reports of NASA, so it is hopeful that the “Earth-like” planet might be possibly discovered in the future with

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1 Introduction: Why Do We Fly into the Space?

Fig. 1.3 Flying into space to look for the alien planets at where human can survive. Konstantin Tsiolkovsky quoted: “Earth is the cradle of humanity, but one cannot remain in the cradle forever”

the help of James Webb space telescope, launched from French Guiana on DEC 25 of 2021, with a most powerful 6.5-m primary mirror for infrared observation. The ancient Chinese philosopher Laozi told us that a Journey of a thousand miles begins with a single step (“千里之行, 始于足下”). This book introduces the free fall, weightlessness, and various phenomena and life in the space station. The book serves as a beginner material of primary popular science for children and public, with basic introduction to the physical understanding of spaceflight.

Chapter 2

Free Fall and Weightlessness in Gravity

2.1 The Story of Isaac Newton and the Apple Falling Gravity, also named as gravitation, is a physical phenomenon of pulling force of two objects with masses, which has existed since before the birth of humankind or even the Earth, or exactly the gravitational force occurs with the birth of the universe dated back to 13.8 billion years ago. The law that governs the gravitational pull between objects was discovered more than three hundred years ago, and is ascribed to a physicist in England, named Isaac Newton. Legend has it that an apple fell on Newton’s head while he was resting under a tree. Most people would probably feel upset by such misfortune, but it made Newton wonder: Why did the apple fall from the tree? Why didn’t it fly up into the sky? After a serial of considerations, he concluded that there would have to be a type of force that was directed towards the center of the Earth. Then Newton began to wonder why the moon didn’t fall and crash into the Earth. He realized that the motion of the moon in its orbit created a force to counteract gravity, and that this principle could be applied to all objects in the cosmos, such as the planets in the solar system. One could say that Newton unraveled the mysteries of astronomical objects from a falling apple (Fig. 2.1). In fact, we are lucky that an apple fell on Newton’s head! Many people could have apples falling on their heads, but few would want to think about why this happens. Newton, however, loved exploring questions with his mind, and he became one of the greatest physicists and discovered the law of universal gravitation, which has governed the whole classical physics for more than three hundred years. This is an example of a scientific mindset and the spirit of exploration. Therefore, students should be challenged to be curious, explore deeply, and be given the opportunity to become the great physicists.

© Springer Nature Singapore Pte Ltd. 2023 C. Zhang, An Adventure, https://doi.org/10.1007/978-981-19-9221-6_2

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Fig. 2.1 The story of Isaac Newton (1643–1727) about the gravity of the apple and moon motion

2.2 Why Does a Cannonball Fall Down to Earth but the Moon Does Not? Newton further speculates that if the moon is like an apple, why doesn’t the moon fall down to the Earth? Is this because of the motion of moon? Then, a series of ideas emerged. When we shoot an arrow with a bow, it will soon fall back to Earth, on the order of a hundred meters from us; when we fire a cannon gun, the cannon ball can travel as much as several tens of kilometers. Because of gravity, arrows and cannonballs will fall back to the ground, but why does the moon stay in the sky? The moon moves around the Earth with a very high speed, producing centrifugal force which overcomes the Earth’s gravity to keep the moon in its regular orbit. Do you know how fast the moon is moving relative to the Earth? Its speed is 1.023 km/s at the distance of 380,000 km away from the Earth, such an unimaginable speed in our normal human life. As an comparison example, a high-speed train travels at about 350 km/h, which is only about 100 m/s. The six-time Olympic gold medalist, Usain Bolt, runs at a speed of 10.438 m/s, based on his running record of 100 m in 9.58 s. In order to escape from the Earth’s surface, at where the gravity is much stronger than that on the moon since the gravitational force gets weaker with the longer distance, he has to reach the speeds of 7.8 km/s, or 7800 m in one second, therefore he will need a rocket to realize this fantastic dream!

2.3 Why Do Scientists Launch Spacecrafts from the Equator?

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Fig. 2.2 If a golf or football were kicked into the space with a velocity of 7.8 km/s, it would circle around the Earth as a moon does

The reason for the stationary orbital motion of the moon is that the gravitational pull of moon-Earth is just balanced by the centrifugal force that is produced by the circular motion of the moon. If the Earth’s gravity decreased or vanished, then the moon would fall down or crash into the Earth. Fortunately, the velocity and distance of the moon relative to Earth are just right for a stationary state (Fig. 2.2). Generally, a cannon ball has a muzzle velocity of about 1000 m per second, three times the speed of sound (~330 m/s), and a range of 50 km can be reached because of the Earth’s gravity. Thus, it is impossible to fly off the Earth, unless its muzzle velocity reaches a critical value of 7.8 km/s.

2.3 Why Do Scientists Launch Spacecrafts from the Equator? My dear friends, do you know that many countries have built the space launch sites near the equator of Earth? The closer you are to the equator, the better you are able to use the centrifugal force of the Earth’s rotation when satellites are launched. The rotation of the equator itself can offset some of the gravitational pull, so that we could consume less fuel, make the rocket go further, and reduce the overall cost of space launch. At the same time, the launch vehicles can be transported to the launch site by water, while the spent rocket can safely fall into the sea after launch, reducing the risk of accidents. My dear students, did you imagine that there were so many scientific secrets behind the selection of a launch site? (Fig. 2.3).

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Fig. 2.3 The illustration of the space launching site of China near the equator, located in Wenchang, Hainan/China. Credit China Xinhua news agency

When launching spaceships, we can employ the Earth’s rotation to minimize the energy needed by the rocket. The Earth revolves around its axis, and the angular velocity is constant, so the lower the latitude on the Earth, the bigger the rotational radius and the faster the linear velocity. The centrifugal force obtained by a moving object is proportional to its linear velocity, so the bigger the rotational radius, the better the launch site. For example, Europe’s Spaceport in French Guiana, at 5° latitude North, is near the equator; NASA’s Cape Canaveral is located in Florida, at about 28° latitude North; China’s Wenchang Spacecraft launch site is on Hainan island near the South China sea, at about 19° latitude North. The geographic latitude was an important consideration for all of these launch sites. Note A lot of satellites, e.g., James Webb Space Telescope of NASA, have been launched from the Arianespace launch complex at the European Spaceport located in French Guiana. At launch sites near the equator, the rotation of the Earth can give an additional push to the rocket. The velocity of the Earth’s surface by its rotation at the equator is 0.436 km/s

2.4 Weight and Weightlessness

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2.4 Weight and Weightlessness If a polar bear weighs 40 kg on the ground, how much does it weigh in a moving elevator? When the elevator speeds up, the bear weighs 60 kg; when the elevator falls freely, the weight of the bear becomes 0 kg. How is this possible? (Fig. 2.4). The weight of the human body is generated by the Earth’s gravity, and what we usually call “weight” corresponds to the force of a person standing on the surface of the Earth. If we are in a moving elevator, we feel heavier when the elevator moves up, because gravity and the acceleration join forces. On the other hand, when the elevator moves down, the balance between the two forces can make us feel weightless. Similarly, astronauts feel very heavy when they are launched into space, and feel weightless when they fall back to Earth. Spacecrafts are in a roughly circular orbit around the Earth. As the human body’s gravity and centrifugal force introduced by the circular motion offset each other, astronauts feel no gravity and weightlessness. However, the mass of the human body didn’t disappear, since the mass of the body is composed of the number of neutrons in the body of the astronaut. We need to distinguish between the two terms “weight” and “mass”; your weight can change that depends on the gravity environment, but your mass remains the same forever.

Fig. 2.4 If a bear stands on the ground of elevator in stop state, it weighs 40 kg; while the elevator freely falls down, the weight scale indicates a 0 kg, no weight at all. The bear loses its weight in free fall state

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2.5 The Wonderful World of Microgravity Microgravity means that the gravitational force on the object is very small, not more than one ten thousandth of the weight on the Earth. The space station is a microgravity environment where the centrifugal force and the Earth’s gravitational force are almost balanced. We want to stress that the gravity of the Earth still exists on the orbiting space station, and the weightlessness does not imply the absence of gravity! (Fig. 2.5). Astronauts in space have to adapt to their weightless state when going about their daily life. Any object that is not strapped down will keep floating. Astronauts have to push themselves off the walls to move their bodies. Many cabin activities require special equipment. In the weightlessness of space, human limbs have no sense of weight, and the body cannot feel the head. These unusual feelings cause the astronauts to experience the directional illusions. When the bulkhead is pushed or pulled by astronauts, they don’t think they are moving forward but rather that the spacecraft is moving backwards. In a microgravity environment, every astronaut looks like a martial art master. They can easily accomplish actions which are incredibly difficult or impossible to do on the ground. For example, one can easily lift another astronaut and flip him or her in the air! On the other hand, such an abnormal environment can also cause dizziness, nausea, sleepiness and the body’s organs will also be affected. Due to the lack of gravity, a large fraction of the body’s blood will be transferred to the upper body and head. Astronauts may experience bulging neck veins, puffiness of the face, nasal and sinus congestion, and other abnormal phenomena. The transfer of body fluids will cause astronauts to reduce plasma volume, leading to anemia. For long duration space flight, medical research will have to find ways to mitigate the effects of weightlessness on the human body.

Fig. 2.5 The astronauts fly freely in the space station

Chapter 3

Measure the Body Weight on Space Station

3.1 Measure the Weight on the Ground In space, astronauts must pay even more attention to their health and physical conditions than we do on Earth. How do astronauts measure their body weight on the space station? Can they use a spring scale or balance like they do on the ground? Let’s recall a classic example of measuring weight on Earth. Almost two thousand years ago, an elephant was presented as a gift to the royal court in China. The royal ministers were unable to settle the question of “How much does the elephant weigh?” No scale in all of China was big enough to hold the elephant. Cao Chong, the 6-year-old son of the prime minister, came up with a scheme to weigh the elephant. First, he had the elephant stand in a boat, and made a waterline mark on the side of the boat. After the elephant had been herded back onto land, the boy placed stones in the empty boat, which now floated much higher in the water. He continued piling stones in the boat until the waterline mark was once again at the level of the water. Because each individual stone weighed much less than an entire elephant, their weights could be measured and added together to calculate the weight of the elephant. This is the famous story of “Weighing the Elephant” In the ancient China. The method exploited in this story is similar to the use of balance scales. The main idea of this method is to determine how many stones of known weights are equal to the unknown weight of an elephant. Gravity pulls equally on both the piles of stones and the elephant, so they are in balance. In modern times, we normally use an electronic or spring scale to easily measure a body weight. These scales determine the weight by measuring the additional force exerted by gravity on the scale’s spring when an object is placed on it. The force of gravity pulling toward Earth’s center is balanced by the equal and opposite restoring force of the coiled spring. A spring scale yields a meaningful measurement because the elasticity of the spring has been calibrated with respect to the strength of gravity on Earth’s surface. However, in a space station orbiting Earth, there is no feeling of gravity because it is in free-fall—constantly falling around the Earth. So, the balance © Springer Nature Singapore Pte Ltd. 2023 C. Zhang, An Adventure, https://doi.org/10.1007/978-981-19-9221-6_3

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3 Measure the Body Weight on Space Station

and spring scales won’t work in Earth’s orbit, because all objects there lie in the state of free falls. In space station there is no weight at all, therefore, a different method using a spring can be applied to measure not the weight of an astronaut’s body, but a related quantity, the mass of the body.

3.2 Measure the “Weight” or Mass of Astronaut on Space Station In principle, the procedure for measuring an astronaut’s mass in space is described as follows. One astronaut secures himself to the wall of the spacecraft by holding onto a bracket. A second astronaut stretches a spring with one end fixed at another location and attaches it to the first astronaut. The first astronaut then releases his hold on the bracket, and is pulled by the stretched spring. The second astronaut measures the amount of time it takes for the spring to pull the first astronaut a predetermined distance. This information allows the calculation of the acceleration, a, of the astronaut (that is, how rapidly the spring increased the astronaut’s speed as it pulled him). Because the elasticity (the “spring constant”) of the spring is known, the force, F, applied to the astronaut can also be calculated. Newton’s second law of motion, F = ma, can then be used to determine the mass, m, of the astronaut (Fig. 3.1). In practice, astronauts on the International Space Station use the Space Linear Acceleration Mass Measurement Device (SLAMMD) to perform this procedure. It uses a video camera and computer to measure the acceleration of an astronaut pulled by a system of two springs. One astronaut alone can measure her own mass with the SLAMMD. A similar device was demonstrated by Chinese astronauts on China’s space station as part of a physics lesson broadcast on living TV to students. This measurement method employs springs like a spring scale on Earth, but cannot rely on gravity to stretch the springs in space. How does this work? Imagine two identical springs on Earth, both with one end attached to a horizontal rod. When objects with different masses are attached to the other end of the springs, they will stretch to different distances. It is a matter of common experience that the spring with a more massive (“heavier”) object attached to it will stretch longer. However, if this experiment is repeated under free-fall conditions in a space station, then the springs will not stretch at all when the objects are attached, because there is no net force of gravity to “pull” on them. Instead, if the two identical springs are manually stretched by the same amount and released, then the acceleration imparted to the attached objects will be different. The more massive object will rebound more slowly compared to the less massive object. That is, the spring produces a smaller acceleration for a more massive object. Let’s think about it in another way. Imagine a big truck and a small sport car, both with the same engine horsepower. Which vehicle can change its speed (accelerate) faster? Again, common experience tells us that the sport car can accelerate faster than

3.3 Why Do We Confuse Weight and Mass?

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Fig. 3.1 Illustration of measuring the mass of astronaut on space station by means of the spring

the big truck. Because the truck has a much larger mass than the sport car, it requires much more force to accelerate to the same speed. This is another example of using Newton’s second law of motion to determine the mass of an object by measuring its acceleration due to a known force.

3.3 Why Do We Confuse Weight and Mass? Although the terms mass and weight are often used interchangeably in our daily life, they actually refer to the different things. Mass is an intrinsic property of matter. The mass of an object can be thought of as a measurement of the total number of small particles of matter that make up that object. We might count the atoms of every element in the object (hydrogen, carbon, sodium, iron, and so on), or even tally the smaller subatomic particles from which all elements are formed (protons, neutrons, and electrons). Unless the physical structure of an object is changed (that is, some atoms are added or removed), the mass of an object never changes.

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Weight, on the other hand, is created by the interaction between matter and gravity. Although the mass of an object does not change, its weight will change depending on the local strength of gravity. For example, because the mass of the moon is much smaller than that of Earth (just a little more than 1% of Earth’s mass), the force of gravity on the surface of the moon is smaller than on the surface of Earth. Thus, the same object will have a smaller weight on the moon than on Earth (about 16.6% of its Earth weight), even though its mass has not changed. Bathroom scale reports the weight of an object by measuring the additional force exerted on the scale due to gravity when an object is placed on it. A bathroom scale that reports your weight in kilograms (which is a unit of mass) is actually measuring how much the mass of your body is pulled by Earth’s gravity. Then, using the known, average strength of gravity on Earth’ surface, the scale converts that measurement of weight to one of mass. Because we usually talk about the weight or mass of an object only when it is located on the surface of the Earth, these terms can be used interchangeably. But if we are comparing the mass and weight of an object on Earth to the same object on the moon (or elsewhere), then the two quantities will no longer be the same. In fact, careful measurements of mass and weight of an object in different locations on Earth’s surface can even be slightly different!

3.4 Mass and Weight from North Pole to Equator The mass of an object doesn’t change with its shape, state (for example, gaseous, liquid, or solid), or spatial position. But weight is a sort of measurement of the local gravitational force, so it does depend on the location at which it is measured. For example, because the gravitational force exerted on objects by Earth depends on distance from the planet’s center, the same object will weigh slightly less at the top of a tall mountain than that at the beach. More generally, Earth’s shape is not a perfect sphere. Instead it bulges out slightly at the equator (by about 0.3%, equivalent to 20 km). Because an object at the equator is slightly farther from the center of Earth than an object at the poles, the force of gravity experienced by an object at the equator is slightly smaller than that experienced by the same object near Earth’s poles, by about 0.5%. Consequently, a polar bear on vacation at the equator weighs less than the same polar bear at home at the North Pole! However, the distance from the center of Earth is not the only factor in affecting gravity and weight on Earth’s surface. The rotation of Earth also affects the force that an object experiences (that is, the effective gravitational force). From the point-ofview of an object on Earth’s surface, the force of gravity pulling it down is counteracted by the ground pushing up against it. If an object is located at a pole, then these two forces are exactly in balance, so the object rests on the ground without moving. However, the situation is different when the object is located closer to the equator. In this case, the object is traveling on a circular path through space as it rotates with Earth. Like spinning a rock on the end of a string, this rotation causes a centrifugal

3.4 Mass and Weight from North Pole to Equator

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force to act on the object, directed away from Earth’s center (that is, in opposition to gravity). The circumference of the circular path increases from zero at Earth’s poles (where there is correspondingly no centrifugal force) to a maximum at the equator. Hence, the effective force of gravity is further reduced by about 0.3% at Earth’s equator compared to the poles. Mass and weights of a polar bear in different locations North Pole

Beijing

Equator

The Moon

Mass (kg)

700

700

700

700

Weight (“kg”)

700

698

694

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Taken together, this 0.8% reduction in the effective force of gravity means that a polar bear which “weighs” 700 kg at the North Pole would only “weigh” 694 kg on the Earth’s equator! However, its mass is still the same in both locations. Similarly, while a 75-kg person would probably not notice the corresponding weight change of only 0.6 kg, for a rocket with a mass of more than 500,000 kg, like the SpaceX Falcon 9, it amounts to a difference of more than 4000 kg. This effective reduction in the weight of the rocket means that less fuel is needed to reach orbit or more cargo can be launched into orbit. This is why launch sites for rockets tend to be situated at low latitudes, as close as possible to Earth’s equator (Fig. 3.2).

Fig. 3.2 For a same mass, we measure the different weights at the different places, such as North Pole, equator, and moon

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3.5 How is Earth’s Mass Measured? Much like in the tale of weighing the elephant, there is no scale large enough to measure the mass of Earth. In any case, as discussed above, it would not be possible to weigh Earth in the absence of Earth’s own gravity to make a spring scale or simple balance scale function properly. However, Isaac Newton showed (in his Universal Law of Gravitation) that the force of gravity depends on the masses of both objects being considered. This is not normally obvious because the masses of everyday objects are so small compared to that of the entire planet. Earth’s gravitational pull on objects and ourselves is what we notice, while the infinitesimally smaller gravitational pull we exert on Earth goes unnoticed. However, this dependence on both masses means that if we can measure the gravitational force between Earth and an object of known mass, as well as between the two objects with known masses, then we can derive the mass of Earth. The British scientist Henry Cavendish (1731–1810) performed an experiment to measure the Earth mass, the idea of which was originally considered by another British scientist John Michell (1724–1793). The Cavendish apparatus employed to weigh the Earth was a torsion balance, in which two lead balls of 0.73 kg were suspended at the ends of a rod. Two massive stationary lead balls (158 kg) were arranged nearby. The gravity between the small and large lead balls cause the suspended rod to rotate. By measuring the amount of the rod movement that induced the wire twisting, Cavendish presented the gravity between the two lead balls, and derived the gravitational constant. With the known weights of the lead balls, he also calculated the average density of Earth; after calculating the size of Earth, its mass was obtained. Cavendish originally calculated the mass of Earth to be about 5.94 × 1024 kg, which is very close to the modern measurement value of 5.965 × 1024 kg.

3.6 Short Summary on Mass and Weight The mass is the amount of substance in an object, while weight is a measure of the pull of gravity on the object. Weight depends on the strength of the gravitational force where an object is located, but mass doesn’t change regardless of location. Weight is a consequence of the balance between the pull of gravity on an object directed toward the center of a planet, and the equal and opposite force exerted upward on the object by the surface of the planet. You can only use the terms mass and weight interchangeably when discussing objects that are all subject to the same gravity, for example, on the surfaces of Earth or the moon (but not in comparisons between Earth and moon).

3.6 Short Summary on Mass and Weight

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The weightless environment experienced by astronauts in orbit around Earth does not imply the absence of gravity, just that they are accelerating freely in their orbital path at the rate produced by gravity at their orbital distance from Earth’s center. They are in free-fall around Earth, on a path that never collides with Earth’s surface. Despite the sensation of weightlessness in orbit, the mass of an astronaut is the same as on the surface of Earth, or the moon, or anywhere else.

Chapter 4

The Space Swing Never Stops

We have commonplace experiences of simple pendulum motion on Earth; for example, the swinging weight that keeps time in a grandfather clock or the swings that are popular with children on playgrounds. Both of these are examples of a pendulum. But how might this familiar behavior be affected by the weightless, freefall environment inside an orbiting spacecraft? To start with, let’s explore how a simple pendulum operates on Earth (Fig. 4.1).

4.1 Constructing a Simple Pendulum A simple pendulum can be easily constructed by suspending an object from a string. The suspended object is called the bob; a small but massive object works best, like a metal ball. The string is called the rod; it can be a light string or even a thin metal wire or literal rod. For best results, the mass of the rod should be much smaller than the mass of the bob. One end of the rod is attached to the bob, and the other end is attached to a pivot on a support structure, such that the bob can hang down and swing freely. The act of pushing the bob starts the simple pendulum motion—Earth’s gravity does the rest. The bob is constrained by the rod to travel in a curved path, forming an arc of a circle centered on the pivot. At first, the bob moves upward in its swing, its velocity slowing due to the downward force of Earth’s gravity. When the upward velocity of the bob reaches zero, it begins to swing downward, accelerating due to Earth’s gravity. At the bottom of the wing, the velocity of the bob is the same as it was when it was pushed to start the motion, if there is no friction between the rod and pivot. It swings upward on the other side, again slowing due to Earth’s gravity, and the process repeats. In the absence of other forces acting on the pendulum (for example, air resistance, friction in the pivot, and so on), the pendulum’s swing would continue indefinitely.

© Springer Nature Singapore Pte Ltd. 2023 C. Zhang, An Adventure, https://doi.org/10.1007/978-981-19-9221-6_4

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Fig. 4.1 Illustration of swing pendulum on the ground of Earth

4.2 A Pendulum Anchored in the Cosmos In 1851, the French physicist Léon Foucault (1819–1868) placed a pendulum device in the Paris Observatory, and in the Paris Panthéon later. The rod of this pendulum was a very long, a thin wire of 67 m in length, and the bob was a brass-coated lead ball with a mass of 28 kg. The pivot allowed the pendulum to swing, as well as rotate. A pin attached to the bottom of the bob made marks in a layer of wet sand under the pendulum, allowing its progress to be monitored. Foucault’s intent was to demonstrate Earth’s rotation. The heavy bob of the pendulum helped to minimize the effect of external forces (air resistance, friction), while the long rod attached to the ceiling of the Panthéon dome allowed the pendulum to effectively be anchored in “space” over the surface of Earth. After a period of time, one would observe the ground under the swinging pendulum rotating due to Earth’s rotation. This experiment showed that the moving plane of the pendulum’s swing rotated from east to west, corresponding to the expected west to east rotation of Earth. This was the expected direction because the Sun, moon, and other celestial objects are

4.3 The Mystery of Swinging

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Fig. 4.2 Illustration of a playground swing around a pivot

observed to apparently rise in the east and set in the west, requiring that the underlying orientation of Earth’s rotation—the true source of the rising and setting—has the opposite orientation. Surprisingly, Foucault’s pendulum verified Earth’s rotation. In Paris, the plane of the pendulum’s swing completed a full rotation in 31.5 h. At the north or south poles of Earth, a Foucault pendulum would complete a full rotation in 24 h, the length of Earth’s rotation. At lower latitudes, the rotation period of the pendulum increases until, at the equator, the pendulum does not rotate at all. In fact, the observed rotation period of a Foucault pendulum can be used to determine the latitude of the site where it is located (Fig. 4.2).

4.3 The Mystery of Swinging The swings in a children’s playground are also pendulums. When the swing is pushed or pulled up and released, gravity takes over and the back and forth swinging motion of a pendulum begins. But as every child knows, if you do nothing, then the swing will soon stop swinging. This is because the pivot of the swing is subject to friction, and the bob—in this case, a child seated in the swing—is subject to air resistance. These factors “steal” energy from the motion of the swing, causing it to slow down and decrease the width of its back and forth motion.

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But every child also knows how to make the swing keep swinging, and even swing higher. Lean back and stretch your legs out straight on the forward swing, then sit up and tuck your legs under on the backwards swing. The sudden change in orientation of your body (the bob of the pendulum) at the high point of each swing adds energy to the pendulum, resulting in a faster speed through the low point of each swing. Repeating this “pumping” process not only counteracts the losses caused by friction and air resistance that would cause the swing to slow to a standstill, but enables the swing to reach higher and higher at the end of each swing. Where does this extra energy come from? It’s provided by your muscles! Reversing this process (sit up on the forward swing, lean back on the backwards swing) removes energy from the pendulum and will cause the swing to quickly come to a rest. Of course, you can also just drag your feet or jump out! Another way to pump a swing is to stand on the seat, crouching down at the high point of each swing and standing up at the low point. This moves the mass of your body (the pendulum’s bob) away from the pivot point when the speed of the pendulum’s swing is slowest (at the high points) and toward the pivot point when the swing speed is highest (at the low point). The effect is the same as if the length of the pendulum rod was shortened every time it passed through the low point of the swing, then lengthened as it reached the high points of the swing. Because a pendulum swings faster when its rod is shorter, this adds energy to the motion of the pendulum as it passes through the low point of the swing (when it is moving fastest). The pendulum bob can then rise higher at the end of each swing before the downward pull of gravity slows it to a standstill and reverses the direction of the swing. The physics of this process stem from a property called the conservation of angular momentum, which basically states that the energy of motion of a large object spinning slowly is equivalent to that of a small object (of the same mass) spinning rapidly. This same effect allows an ice skater to start spinning slowly with outstretched arms and legs, and greatly increase the speed of her spin simply by crouching and pulling in her arms. In practice, “pumping” a playground swing is a complex process that combines elements of both of these methods (rapid change in body orientation and changing the effective length of pendulum rod) (Fig. 4.3). Through practice, children quickly master the most efficient method! But can you swing in a spacecraft? On Earth, the restoring force that slows and reverses the swing of a pendulum is gravity. Pumping the swing relies on making changes to the parameters of the pendulum during different parts of its swing (high and low, fast and slow). In a weightless and free-fall environment in orbital space station, there is nothing to stop the forward motion of a swing (ignoring friction and air resistance). Even without pumping, the swing could make a complete circle around the top of the swing set!

4.4 Keeping Time with a Pendulum

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Fig. 4.3 An example of swing: the big loop in gymnastics

4.4 Keeping Time with a Pendulum Although often used as the name of the enormous clock in the north tower of the Palace of Westminster in London, England, “Big Ben” is actually the name of the largest of the five bells in the clock. Like many antique clocks, Big Ben uses the regular swing of a pendulum to keep time. The pendulum swings every two seconds, and a spring and gear system is wound every few days to return energy to the pendulum that is lost due to friction. Small adjustments to the timing of the clock are made by placing old English penny coins on top of the pendulum bob. This raises the center of mass of the 300-kg bob, effectively reducing the length of the pendulum rod. As discussed above with regard to pumping a playground swing, shortening the rod of a pendulum makes it swing faster. Adding or removing one coin on the pendulum bob in the Big Ben clock changes the speed of the clock by only 0.4 s per day (Fig. 4.4). In the case of Big Ben, time really is money! Their usefulness in timekeeping stems from a property of the pendulum called isochronism. This property was first discovered by the Italian physicist and astronomer Galileo Galilei (1564–1642) in the early seventeenth century. He found that the time that a pendulum takes to complete each swing, called its period, depends only on the length of the pendulum and the strength of gravity in its location. For example, the width of its swing measured as an angle in degrees between the lowest point in the center of the pendulum’s swing and the highest point at one end of its swing, called the amplitude, does not matter. A pendulum whose swing follows a long arc will have the same period as an identical pendulum swinging through a short arc. This property of isochronism holds to within a few percent of the period as long as the amplitude of the pendulum is less than about 45°. At larger amplitudes, the isochronism begins to break down, and the period of the pendulum grows longer as the amplitude increases.

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Fig. 4.4 The Big Ben of London shows a case of the pendulum

4.5 Simple Pendulum Motion in Gymnastics The goal of the ring apparatus in modern gymnastics competitions is for the hanging rings to remain as still as possible while the gymnast performs his routine—not like a swinging pendulum at all! But from the early 1800s through the middle of the twentieth century, another form of ring event was also performed, called the “flying rings”. In this form of the sport, the gymnast used the mass of his body to create a simple pendulum by hanging from the rings, suspended on ropes almost 7 m long. After starting with a push from an assistant on the ground, the gymnast could increase the amplitude of his swing by pumping the pendulum like on a swing. He would then perform stunts at the end points of each swing, often reaching heights of 4 or 5 m above the ground. Performances of the flying rings eventually fell out of favor—at least partially because of the element of danger involved for the gymnast—leaving only the still rings practiced today. A gymnast swinging from her hands on a horizontal bar also behaves like a pendulum. By pumping her swing, she can gradually increase both her speed through the low point of the swing and the amplitude of the swing. By accumulating enough

4.6 A Pendulum in Weightlessness State

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Fig. 4.5 A pendulum example: big loop of horizontal bar, in gymnastics

speed, she can even make a complete loop around the horizontal bar. A pendulum with a flexible rod will not normally be able to do this. Once the amplitude of the swing reaches much more than about 90° (that is, more than one-quarter of a circle from the bottom of the swing), gravity can make the pendulum’s bob fall straight down when it is at the high point of the swing and has the lowest speed, interrupting the periodic back and forth motion of the pendulum. The gymnast can swing with arcs larger than 90°—even in a complete circle— because she uses her muscles to keep her body rigid and to add energy to her swing. Both of these factors counteract the pull of gravity that dominates the behavior of a simple pendulum. Her body starts to behave less like a pendulum and more like spinning a rock on a string or a spacecraft orbiting Earth (Fig. 4.5).

4.6 A Pendulum in Weightlessness State On Earth, we start a pendulum swinging by applying a force; for example, by pushing or pulling the bob and releasing it. When the force is no longer applied, friction and air resistance cause the swing of the pendulum to gradually decrease in amplitude until the pendulum stops swinging. But in the case of an “ideal pendulum” that is frictionless and not subject to air resistance or other losses of energy, the bob would continue swinging indefinitely once started. But what would a pendulum do in an orbiting space station? Gravity makes a pendulum work; in a free-fall, weightless environment where gravity has been effectively negated, we should expect that the behavior of the

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pendulum will be greatly affected. First, the pendulum bob would tend to rest (“float”) wherever it was released rather than settling to an equilibrium resting position hanging straight down at the lowest point of its swing. Applying a force to the bob like we would do to start the pendulum swinging on Earth would simply move the bob in a straight line until it reached the end of its string. Without the force of gravity to supply the initial tension in the string and the means to trade the energy of the pendulum between energy of motion at the bottom of the swing and potential energy at the top of the swing, the pendulum will not work. Applying a large force to the bob can provide enough tension in the string to make the pendulum swing in a complete circle around its pivot, like quickly spinning a rock on a string on Earth. This is an advantage of operating the pendulum in free-fall: gravity does not alter its initial speed at all, so the pendulum bob can continue in circular motion around the pivot until friction losses slow it down. In this way, a pendulum in space acts like an orbiting spacecraft, eternally circling Earth. But the properties of a pendulum that require gravity will not work in orbit—so there’s no Big Ben clock and no playground swing in space station (Fig. 4.6). Fig. 4.6 Swing of pendulum makes a big loop and never stops in the space station

Chapter 5

The Gyroscope on Space Station

5.1 Stability of a Spinning Gyroscope Gyro toy for children has existed for thousands of years. Examples are found in archeological artifacts from many societies around the world. The pointed, symmetric shape of the top, like an inverted cone, allows it to balance on its tip while spinning due to its rotational inertia. The rotational inertia of an object depends on its size, shape, and mass, and is related to its angular momentum, which is the product of the rotational inertia and the rate of the object spins (Fig. 5.1). Unless acted on by an unbalanced force (such as friction between the top’s tip and the table, or gravity if the top begins to wobble), rotational inertia and the conservation of angular momentum makes a spinning object continue spinning at the same rate and orientation. This is related to the spinning ice skater in relation to the motion of a pendulum. When the skater decreases her rotational inertia by pulling in her arms and legs (that is, changing the shape of her body), the speed of her spin increases in order to conserve (that is, keep constant) her angular momentum. Because the rotational inertia (that is, shape, size, and mass) of a spinning top does not change, neither does the speed and orientation of its spin. A gyro top can be spun between the child’s fingers or “whipped” using a stick or string. A related toy is the diabolo, which is a juggling prop consisting of an axle with two attached cups or disks in an hourglass shape that is spun using a string attached to two handheld sticks. It maintains its spinning motion based on the conservation of angular momentum. In China the diabolo is a traditional folk activity, with the implements often made of bamboo and called “fast rotation of the ballet” and so on. A gyroscope is essentially a top mounted inside a series of three concentric gimbals. A gimbal is a support ring that can pivot freely around one axis. In a gyroscope, the nested gimbals are mounted such that their axes are orthogonal (that is, at right angles) to each other. The spin axis of the top is mounted in the inner gimbal ring, which is mounted to the middle gimbal ring on a spin axis rotated by 90°. The middle gimbal is mounted in the outer gimbal ring on a spin axis rotated by an additional 90° (resulting in a total of 180° relative to the spin axis of the spinning © Springer Nature Singapore Pte Ltd. 2023 C. Zhang, An Adventure, https://doi.org/10.1007/978-981-19-9221-6_5

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Fig. 5.1 Illustration of gyro toy

top at the center—this means that the spin axis of the top and the outer gimbal are aligned in the same direction). This arrangement allows the spinning top to freely change orientation in three dimensions. In practice, the spinning top in a gyroscope is replaced by a disk mounted on a thin axle (called the rotor) that passes through the center of the disk in its narrow dimension. From an engineering point of view, a gyroscope is an object that rotates about its axis of symmetry. The rotational inertia of this shape enhances the degree to which the plane of the gyroscope’s spin resists change. Hence, the spin of a gyroscope is very stable. If the orientation of a gyroscope is changed, the spinning disk within the movable frame and gyro will reorient itself to its original direction of spin. In fact, the most important characteristics of a gyroscope are its fixed axis and stability of spinning (Fig. 5.2). In practical circumstances, it is almost impossible to avoid all interference with the motion of an object like a gyroscope. If a gyroscope is affected by a small torque (that is, a force that is applied unequally relative to the center of the gyroscope), then the gimbals will change orientation. The rotor will reorient itself to its original plane of spin, with its spin axis pointing in the original direction; however, the pivot axes of the gimbals will point in different directions. That is, although the orientation of the rotor will not change, the gyroscope will be “tilted” relative to the original Fig. 5.2 Illustration of gyro toy: the diabolo

5.1 Stability of a Spinning Gyroscope

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orientations of the gimbals. The gyroscope will undergo precession, in which one end of the outer gimbal’s pivot axis will remain fixed while the other end moves in a circular path around the original pivot axis of the outer gimbal. The precession of the gyroscope will continue indefinitely as long as the torque continues to be applied to the gyroscope, causing it to tip away from its original orientation. On Earth’s surface, the torque in a gyroscope that is initially repositioned manually can be maintained by the force of gravity acting on the tipped gimbal frame. Manually restoring the gyroscope to its original orientation removes the gravitational torque and the precession would stop. This example ignores the input of friction in the rotor and gimbals, which can also produce torques over time that cause the gyroscope to process. Minimizing this unwanted motion of the gyroscope is achieved by using a relatively massive, rapidly spinning rotor, and ensuring that the rotor and gimbals can spin as freely as possible. The gyroscope’s fixed axis has been widely used in many technical fields. For example, imparting a high rate of spin of a projectile in a direction orthogonal to its trajectory causes the projectile to act like a gyroscope. This stabilizes it while in flight and can lead to a longer distance traveled with a more precise trajectory. Spinning a spacecraft in orbit will gyroscopically stabilize it, minimizing the need to use attitude control thrusters to maintain its orbital trajectory and orientation in the direction along the spin axis. Especially in modern navigation technology, the use of gyroscopes has achieved much progress, so that navigation has entered a new era (Fig. 5.3). Fig. 5.3 Orientation of spinning gyro is stable towards a remote star direction, e.g., Polaris, which can be applied to navigate the aircraft and satellite

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5 The Gyroscope on Space Station

Fig. 5.4 Illustrations of gyroscope in the states of non-spin (upper, unstable) and spinning (lower, stable) in space station. The spinning gyro is stably orientating to its initial direction

5.2 Gyroscope in Weightlessness How will a gyroscope behave in the freefall (“weightless”) environment of an orbiting space station? When the gyroscope rotor is not spinning, and an astronaut exerts a force on one end of the spin axis by pushing it with her finger, the gyroscope will simply roll and tumble, with its spin axis constantly changing direction. This is the same behavior that any object would display in such a circumstance. When its rotor is not spinning, a gyroscope has no special properties! But when the astronaut pushes the end of a spinning gyroscope’s axis, while the entire gyroscope might move (“float”) around in the space station environment, its spin axis will not change direction. Thus, spacecraft engineers utilize gyroscopes to internally track the motion of a spacecraft (by monitoring the orientation of a gyroscope in a freely-rotating set of gimbals), stabilize a spacecraft (using a system of at least three gyroscopes rigidly mounted in orthogonal directions), and change the orientation of a spacecraft (by using a motor to tilt one of the gimbal mounts, thereby causing the rigidly mounted gyroscope assembly to exert a torque on the spacecraft). Orbiting space telescopes use gyroscopes to achieve the very precise and smooth pointing and tracking movements (attitude control) necessary to make stable and accurate observations of astronomical objects (Figs. 5.4 and 5.5).

5.3 Gyroscopes in Our Daily Life Many commonplace applications here on Earth are related to the unique properties of gyroscopes. For example, the ease with which humans can balance upright on a twowheeled bicycle in motion has to do, at least in part, with the gyroscopic stability of the bicycle’s spinning wheels. What are some other examples of gyroscopic behavior?

5.3 Gyroscopes in Our Daily Life

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Fig. 5.5 Gyroscope is freely spinning and drifting on the space station

(1) Gyroscope example: the non-shaking camera Using a camera while in a moving vehicle (for example, aerial photographs from an airplane or helicopter, or video shot from a moving car) can produce unsatisfactory results, with photographs or videos coming out blurry or “shaky”. High end photographic equipment can utilize gyroscopic sensors to track the motion of the camera, mechanically correct the optics in real time, and thus provide an “anti-shake” feature that preserves image quality. This result is obtained through mechanical means, by physically moving internal optical elements of the camera to counteract the detected motion of the camera. Some modern digital cameras circumvent the need for gyroscopes by using software post-processing to shift the detected light among the pixels of the camera’s solid state detector to minimize jitter and shake (Fig. 5.6). (2) Gyroscope example: spinning of a basketball Although it is not an element of the game itself, the trick of spinning a basketball while it is balanced on a fingertip is an impressive demonstration of the skill of a player. But it really just takes advantage of the properties of a gyroscope. By pointing a finger upward through the center of the basketball, the player defines a spin axis for the gyroscope. When the basketball is spun rapidly around this axis, it behaves like a gyroscope, resisting movement of the spin axis, and easily balancing on a fingertip. Or at least easy in principle! In real life, some practice is likely to be required!

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Fig. 5.6 Gyroscope application: image stabilization in photographs and videos

The mystery of the banana kick Does the banana kick use the principle of gyro? When the football player kicks the football from one side, the ball flights and rotates in the air. Due to the rapid pace and the role of air, football is deformed and becomes an irregular ellipsoid. This will drive the air layer on the surface of the football to rotate at the same time. The linear velocity of the air layer on one side and the advancing speed of the ball are superimposed, which makes the head of the air flow suffer great resistance and slow down. On the other side, according to Bernoulli’s law, the smaller air flow, the greater pressure and this is the principle of the plane taking off. Taking left-lateral football, for example, its line to the left side is lateral force and forms an arc. The path of the football is deflected, and the parabola becomes a banana line, which makes it difficult for the goalkeeper to judge, and this is the secret of the “banana kick” (Fig. 5.7). (3) Gyroscope example: spinning bullet In the times of Napoleon, we may have such a question: why these people have to stand in a row, and behind them there are still two rows of soldiers? Most of the guns in the Napoleonic period were called muskets. The so-called musket is loaded from the muzzle loaded ammunition, gun chamber without rifle before the loaded gun, so there will be two problems: The first one is the slow loading. A skilled soldier can only launch two times in one minute, so three rows of soldiers are needed to turn to launch. The second one is no rifle bore, the bullet shot after the gun is no spin forward, so the stability is lacking and the gun will suffer larger friction in the process of fly so that the flying distance of the bullet is very short, generally not more than 100 m, and the precision is particularly poor. In that times, about every 400 rounds of bullets actually shot the target, so they can only rely on the queue shot to compensate for the poor accuracy problem. And the queue of soldiers and drum musicians are also very calm on the battlefield, war correspondents and photographers can also calmly record the scene of the war, because the probability of being hit by a bullet is really low (Fig. 5.8).

5.3 Gyroscopes in Our Daily Life Fig. 5.7 Illustration of spinning football goes a “banana” route

Fig. 5.8 Illustration of the bullet motion without/with spinning (upward/downward)

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With the advent of rifling guns, this sort of queue shot in the Napoleonic period became the best target, so the tactic was gradually withdrawn from the historical stage. Grooves are the soul of the barrel, it makes the warhead in the chamber rotate in high speed. We’ve learned from the stability of the gyro that rotating objects will maintain their steady state motion. Therefore, rifled gun range and precision are greatly improved.

5.4 Gyroscopes in Universe: Earth and Pulsar To some extent, all spinning objects in the Universe (planets, stars, even galaxies) act like gyroscopes. The gyroscopic behavior of Earth, for example, leads to stabilization of the spin and tilt of Earth in its orbit around the Sun. At the same time, however, the Earth’s gyroscopic behavior is imperfect. Our planet is processing, its spin axis slowly wobbling in a small circle relative to the comparatively fixed positions of distant stars. This precession has a number of consequences, which play out over the approximately 26,000-year interval required to complete one circular wobble of Earth’s spin axis. For example, the seasons on Earth are linked to the tilt of its axis relative to the Sun during Earth’s orbit. When either the Northern or Southern Hemisphere is tilted toward the Sun, that hemisphere experiences summer. The opposite hemisphere is pointed away from the Sun, and experiences winter. The precession of Earth’s spin axis means that the time of year when each season starts (that is, the equinoxes and solstices) will slowly change. As measured by the time of the summer solstice, summer actually begins about 20 min earlier every year! Corresponding to about 1 day every 70 years, this is little comfort for students waiting for school to let out! But after about 13,000 years (one-half of the full precession cycle), the summer and winter solstices will have swapped positions in Earth’s orbit around the Sun. Another example is the North Pole Star, Polaris, which has been used for centuries as a navigational aid in the Northern Hemisphere. But the position of the North Star, as well as the fact that it is relatively bright and easy to spot with the naked eye, are just coincidences! Currently, the north end of Earth’s spin axis happens to point in a direction in space very close to the direction to Polaris. So Polaris always appears in the sky over a region close to the geographic north pole of Earth. But precession is gradually moving Earth’s spin axis relative to the distant stars. Starting at the end of the twenty-first century, precession will cause Earth’s spin axis to move away from its closest approach to Polaris. This means that Polaris will appear to gradually move away from the north pole. Within a few thousand years, Polaris will no longer be the brightest star closest to the north pole. Over the full 26,000-year precession cycle, Earth’s spin axis will point close to other bright stars in the sky. Each of them will take their turn as the North Star (including Vega, the second brightest star in the night sky, about 11,000 years from now), until Polaris resumes that role near the start of the next precession cycle, around the year 27,800.

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The most stable known gyroscopes are naturally occurring astronomical objects called pulsars. Discovered in 1967 by the British astronomers Jocelyn Bell (1943–) and Antony Hewish (1924–2021), pulsars are extremely rapidly spinning, astoundingly dense neutron stars. They are the collapsed remnant cores of stars much more massive than our Sun, which have ended their lives in a supernova explosion. In a neutron star, up to several times the mass of the Sun has been compressed into sphere with a diameter of only thirty kilometers—about the size of a normal city. The mass of one teaspoon of neutron star would be more than 5000 billion kilograms (5,000,000,000,000 kg), about 1000 times the mass of the Great Pyramid of Giza. A piece of neutron star the size of a table tennis ball would be equivalent to a large mountain. The density of a neutron star is comparable to the density of matter in an atomic nucleus, which is part of the reason for their name. Pulsars are neutron stars that spin rapidly, from a few times a second to as many as several hundred times per second (the so-called “millisecond pulsars”). They are observed to “pulse” if beam(s) of energy directed out from the magnetic pole(s) of the neutron star are briefly visible from Earth as the neutron star spins. This requires the twin coincidences that the pulsar’s magnetic field and spin axes are offset from each other, and the orientation of the pulsar is “just right” so that the beam(s) sweep across Earth (like the beam of a lighthouse viewed from a ship at sea). Because of their very high mass and extremely rapid spin, pulsars (especially millisecond pulsars) act like extremely stable gyroscopes. Their spin period (the time required to spin once—measured in thousandths of a second for the fastest spinning pulsars) changes by only the order of a second during an interval of several hundred million years. This is better than the accuracy of any atomic clock ever made! Pulsars also tend to have low “proper motions”, or rate of movement through space. Because of the vast distance scales of the Universe, even pulsars with a speed relative to Earth of several 100 km per second (about 360,000 times faster than freeway driving speed!) are effectively fixed in position on timescales of many millions of years. This makes pulsars like the lighthouses in another way: they are useful navigational aids for locating the position of astronomical objects by their relative distance from several pulsars of precisely known spin periods. This technique can be used and attached to the pioneer space probes to pinpoint the relative location of the Sun for any hypothetical intelligent extraterrestrial beings that might stumble across the probes in the distant future (Fig. 5.9).

Further Reading Like a magnet compass to indicate the direction on the earth, the directional gyro can navigate as an indicator to show the direction in space travel or aircraft. This gyro is a fast rotating object, e.g., at a rate of 30,000 rpm, and it is little influenced by the orbital motion of the space craft. As the space station travels around the earth, the freely spinning gyro remains stationary to point at its

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Fig. 5.9 A pulsar is a stable rotating neutron star, compact object, and its emission pulse signal is like a shining beacon swept our line of sight as it spins, e.g., a round per second. Its stability properties of spinning orientation and temporal pulse can be employed as the navigation instrument in the universe trip

originally set direction, e.g., of Polaris, usually named as the North Star, the bright star near the direction of the Earth’s northern axis.

Chapter 6

Wonders of Water in Weightlessness

6.1 Shape of Water Drop in Weightlessness Droplet shape. On the ground, the water drops are shaped like the outlook of the reverse pear, which is the consequence of gravity and air resistance. The surface tension produces the inward forces on the molecules of the surface water droplet, where the attraction of water molecules by cohesion is greater than that to the air molecules by adhesion, which tends to cause the surface area of the drop as small as possible. In a gravity environment, part of the tension is employed to resist gravity, thus the shape of the drop forms a “thin-tailed oval” shape, or pear-like shape. However, in the weightless condition, what does the water drop look like? If there is no gravity at all, the surface tension toward every direction is homogeneous, and there does not exist the gravity-orientated or preferred direction for the water drop. Therefore, a complete spherical shape of water can be formed on the space station, and in fact an ideal round water droplet on the Earth does not exist. In summary, a water droplet naturally tends to have a minimum internal energy state, and the most stable state of its structure is a sphere that is completely symmetrical and uniform with a minimal area of surface for the same volume. So many other shapes in nature tend to evolve into more stable spheres if there is no gravity (Fig. 6.1). On a space station, it is a weightless environment that makes the water ball shaped as a sphere; because of the surface tension of water, say, water tends to form an ideal sphere there, which is on account of the fact that the water molecules have the tendency to stick together. Without gravitation, the water molecules spread homogeneously to form a round water ball, which is also ascribed to the fact that the surface tension prevents the water molecules from splitting and holds them together. When the external force is applied, the shape of the water ball changes immediately. In principle, an absolute spherical ball can be made in the perfect zero-gravity environment, thus, if we can manufacture the ball bearings on a space station, they will outperform the ball bearings made on the ground. Theoretically, the tidal force

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Fig. 6.1 Comparison of the water drops on the Earth (left panel) and space station (right panel)

also affects the shape of the liquid ball, making it an ellipsoid, but the error that deviated from the sphere is so small that no countermeasure is needed. For a larger celestial body like the moon, the tidal force is large enough that it cannot be ignored. The moon’s face is locked to the Earth by the tidal effect, therefore, only one side of the moon is always facing the Earth. Moreover, the Earth is also affected by the tidal effect of the moon, as the tide of the ocean is the most direct consequence of the tidal force. Strikingly, the space of microgravity is an amazing place full of wonders, and life there is also an attractive and interesting topic, since our human body is composed of a lot of water molecules, the phenomena of which will be discussed in the following contents. Bubble “rising” in weightlessness? On the surface of the Earth, as experienced, if we inject the air into the water tube, or boil the water to produce the vapor, the air bubble would occur and rise upward in the water. Why does it happen? Please follow me slowly to explain the reason. The density of the air bubble is much lower than the nearby water, thus, in terms of the principle of Archimedes (287BCE–212 BCE, Greek scientist), a buoyant force pushes the bubble to rise from the bottom to the top of the water container. The process is described below. Water has a buoyancy effect on the bubble in it because the downward gravity of the bubble is less than the upward buoyancy of water to the bubble, so the bubble will gradually go up. There exists pressure in the water, the deeper the higher, and the shallower the lower. The tension of the bubble is almost constant, and as it rises and moves to a shallower place (top part of water), the bubble volume will expend as the pressure from the shallow

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water decreases. Until the bubble reaches the surface, the water has no pressure on it at all, and the tension causes the bubble to break up, releasing water vapor into air. While, on a space station, the air bubble inside the water will not move at all, since the gravitation has been canceled by the centrifugal force due to circular orbital motion, as a further experiment, if we put a sealed container with the air and water in a weightless environment, the air bubbles will spread around their positions, and there is no rising up phenomenon of the bubble at all, because there is no floating force in weightlessness.

6.2 Thin Film of Water on a Space Station Do you remember that the soap bubbles brought fun for children during their childhood? Inserting one end of the straw into the soapy water, one lifts it up and blows, and the colorful bubbles are formed. It’s wonderful to see the bubbles flying in the air, but bubbles don’t last very long; after a short flight, they will break. Why is it so fragile? Soap bubbles are very thin, hollow-shaped membranes of soapy water that break when they touch other objects or stay in the air for too long, while the Earth gravity thins the film above the soap bubble. How does a thin film of water behave on a space station? The astronaut has made an experiment and presented a robust answer. The astronaut firstly dips a metal ring/frame into the water container, and then takes it out slowly, while a thin film of water is formed (Fig. 6.2). The amazing point is the absence of gravity feeling on the space station meaning that the water film won’t break up easily. Go ahead, the astronaut swings the metal frame a little, and the film does not crack down. The astronaut fixes the frame to a desk, and attaches a knot label to it, as a result, the knot is stuck to the water film. Interestingly, the water film is more stable on the space station than when it is performed on the ground (Fig. 6.3).

6.3 Super Water Globe Ball and Cocktail on a Space Station Super spherical water ball. After making a new water film, the astronaut takes a water bag and begins to inject water into the film with a medical syringe. As the water is added, the water film becomes thicker and thicker. Later on, the thin film becomes a shiny water globe. Carefully looking at it, some air is trapped in the water globe, since the water bag contains the air in it. After sucking the air out with a syringe, the water globe becomes a clear lens, through which we can see the reversed image of the astronaut. Since the water globe is not affected by the weightless environment, its shape is a completely perfect sphere, exactly a super globe (Fig. 6.4).

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Fig. 6.2 Making a thin film of water membrane

Fig. 6.3 A label logo easily sticks to the water film on space station

6.3 Super Water Globe Ball and Cocktail on a Space Station

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Fig. 6.4 On space station, astronaut can make a super spherical ball of water by injecting water into the water film gradually. After injecting the air bubbles inside the ball, these bubbles are cumulated inside the ball and there is no “rising” up movement happening on the ground

The red lantern water ball. Then, the astronaut injects two air bubbles into the water globe. Because of the surface tension, two bubbles do not merge together nor repel out. Then, the astronaut injects the red liquid into the water, and the red color slowly spreads in all parts of the water globe ball, making the globe into a red lantern. This experiment means that the cocktail of wine could not be made on the space station, since the weightlessness raises the liquid to mix evenly, instead of separating into layers because of the liquid densities to form a cocktail on the ground (Fig. 6.5).

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Fig. 6.5 In weightlessness, injections of air bubbles (upper panel) and red liquid (lower panel) into the water balls on space station

No cocktail in weightlessness. We can make a cocktail in our kitchen, where the recipes include water, orange juice, and brown sugar. At first, we dip some brown sugar solution into the tube, then add some orange juice slowly. At this step, we can observe the liquid separation, and find that the orange juice floats above the brown sugar solution. Secondly, we dip some water into the tube, and clearly see the liquid separation occur. From the top to bottom: the water, orange juice, and brown sugar solution are automatically formed in the colorful layer state.

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Fig. 6.6 The cocktail can be made on the ground because of the gravity-induced stratification effect, but it does not exist in the weightless circumstance

The phenomenon of liquid layer separation is caused by the density differential among the brown sugar solution, orange juice, and water. Under gravity, the liquid of various densities separates into layers, forming the cocktail. To successfully make a colorful cocktail, it is essential to know the material density or specific gravity of ingredients, e.g., the densities of water and alcohol are, respectively, about 1.0 and 0.8 g/cubic centimeter. The lighter liquid will set on the top of heavier one, so a qualified bartender must remember the density orders of all brands of beverages. However, on a space station, there’s no gravity feeling, and different liquids tend to mix together because of the Brownian movement of molecules. Thus, the cocktail could not be found and drunk by the astronauts on the space station (Fig. 6.6).

6.4 Amazing Surface Tension The surface of a liquid is like a rubber film, where there exists an intermolecular force generally, named after a Dutch physicist Johannes van der Waals (1837–1923). The van der Waals force decreases quickly as the distance between two adjacent molecules increases. This van der Waals force makes the molecules bind each other, forming the surface tension, which is proportional to the surface area involved. Surface tension exists on the surface of a liquid that shares an interface with a gas (e.g., air), and displays as an elastic film, pulling the liquid particles toward the liquid cluster, binding the surface particles closely. In our daily life, there are lots of examples of

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demonstrating surface tension, for instance, the shape of water droplets and dew on the leaves of plants. Water is easily found everywhere, then, if studied deeply, we can find many interesting properties of it. Why are water drops in approximate round shape, why can objects with a density heavier than water float above it, and what force transports the water from the underground to the tree branches at several tens of meters above the ground? As seen in ordinary life, if too much water is poured into a cup, the water will overflow. However, if we watch the process carefully, we can discover that if the water level is only a little bit over the edge of the cup, the water won’t overflow immediately, while the surface shape is like a balloon. As more water is added, the water surface rises higher and higher, until the surface tension cannot hold the gravity anymore, then the water flows over the edge. It seems that there is a thin film holding the weight of water. Right, this “thin film” produces surface tension. So, how did this surface tension come into being? Water is actually composed of billions of water molecules, each having two basic states, i.e., attracting and repelling. Molecules of water are constantly moving, and they move faster when the temperature rises, and move slower when the temperature drops. If the molecules are crowded too close together, they repel each other, and they attract each other if the molecules separate more apart. In addition, the molecules on the surface tend to vaporize, making the number of molecules on the surface less than the average, so the attraction between molecules forms a net, trapping the water molecules inside. When the gravitational weight of water is greater than the tension force, the water surface will break and flow out. Water tension can explain a lot of phenomena: the water drops on the lotus leaves, water droplets hanging on the faucet, water skippers jumping on the water, and the flying liquid soap bubbles in air. Another phenomenon of surface tension is the capillarity action, which is the attraction of liquid toward a solid surface. Dipping a tube to the liquid, if the liquid sticks to the material of that tube, the liquid level inside the tube will be higher than that outside; otherwise, the liquid level is lower. For example, while two dried panels of glass are put together, it is easy to take them apart. If we dip some water in between the two panels of glass, it is difficult to take them apart. The water and glass attract each other, and it will stick to the glass panel. In addition, paper towels have many small tube structures, which act like glass tubes to absorb the water. So, paper towels can be used to wipe out water on the table. In addition, the tree can transport the water from its root to its leaves, as the fountain pens can move water from the ink bag to the nib (Fig. 6.7). Mosquitoes walk on water. You may have seen mosquitoes walking on the surface of water; so what makes them stand on the water and not sink down? Let’s look into this more carefully; the mosquito extends its legs out, increasing the contact area with the water surface. Also, the whole body of the mosquito is covered with tiny hairs, which trap a thin layer of air, so the mosquito would never get wet. The water surface dips like a rubber film, supporting the weight of a mosquito.

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Fig. 6.7 Illustration of mosquito walking on the surface of water

So, how do mosquitoes keep stable on bumpy surfaces? Mosquitoes have little hairy structures on their legs, which can stick tightly to the surface of water. But those hairs have no use in the water. Those who can walk on the water are the masters of water walking, who rest on the water and wait for the prey. The little hairs on their legs prevent them from getting wet, but mosquitoes don’t have those. Instead, they have tiny holes on their legs, which trap air between the body and water. The surface tension of water prevents those holes from being flooded, keeping the mosquitoes dry. The smaller holes make the water more difficult to go into their leg. Scientists think that the weight the mosquito can support is over twenty times that of its body. When the mosquitoes stand on the water and the water riffles, the surface tension makes them float. However, larger animals like humans can’t stand on the water, since their weights are too much higher than the supporting force provided by the surface tension. The smaller the body is, the more tension it acquires above the water. If we calculate the surface area required for supporting the usual weight of a human, the result would be thousands of square meters, which means that we are too heavy to be walking on water. “Helicopters” on the water surface. Water striders are a common kind of insect, often seen in summer, and they can float, slide, and even jump on water. This amazing skill attracts the attention of scientists. The reason for those skills lies in the ability to repel water, keeping their body clean and dry. More studies showed that water striders have small hairs covering their whole body; those hairs are arranged in the same direction and made of water-repelling material. Mosquitoes can not only walk on water, but also take off and land on water freely. So what is the difference between a water strider and a mosquito? Only experiment and observation can tell. Legs of insects like water striders and mosquitoes are water-repelling. If they are not water-repelling, water would stick to them, making it difficult to move and fly.

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An example is the fly, whose legs doesn’t repel water; so a fly on the water can hardly fly again. But for water striders and mosquitoes, water cannot wet them. In short, the insects can walk on water, because their legs can make the surface of water depressed, without breaking through the water surface, to create the tension forces to support their weight (Fig. 6.8). Amazon lizard running on water. In the rainforest of Amazon, Brazil, there is a dwarf gecko, one of the smallest animals, whose body is only two centimeters long. The four craws of a gecko have suckers on them, and won’t break the surface tension of water, so the dwarf gecko could run on the water to capture the other water-walkers, such as mosquitoes. When we talk about lizards, we always think of them living on deserts and around volcanoes. But actually, most lizards live near the water, and if the humidity is too low, lizards won’t stand too long. Recently, scientists found a special kind of lizard that is able to run on the water surface, so let’s learn about it. Recently a sharp camera, with footage of 2000 frames per second, shows that a brown basilisk lizard is running fast on the surface of a lake. The other video footages Fig. 6.8 Amazon lizard can run on the water, not by the floating force but by the surface tension of water

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show that this lizard is very small that won’t break the surface tension of the water, so that it can walk across streams. Basilisk lizard is often called the “Jesus lizard”, and we can see why it can walk on water. Many insects have similar skills, but they are often small and light and won’t break the surface tension. Basilisk lizards usually live in the rainforest near a river, living on little insects. When danger comes, the basilisk lizard will jump into the water and run away on its surface, putting other predators and natural enemies at risk. People living near water can swim, so animals living near water are also more skillful in water. Thinking in this way, maybe we can understand why animals can live on water. Coin floating on water? Gently putting a coin in water, there’s a chance for the coin to float on the surface. This is also caused by surface tension. Watching carefully, we can discover that the water surface is sunken like a rubber film, which is exactly what made the coin float. When doing this experiment, there are some points to remember: the coin should be parallel to the water surface, and the hand needs to be steady and gentle. After a few practices, you should be able to float a coin on water. Actually, every liquid has some sort of surface tension, only different in magnitude. Sometimes, we can see mosquitos standing on water and not drowning, which is because the pressure mosquitoes apply to water is less than the limit of surface tension (Fig. 6.9). Stone skipping. Have you played stone skipping when you were kids, which has a little to do with surface tension? Throwing a small stone horizontally into the water, the stone would skip and jump on the water several times. Furthermore, do you want to know the pro tips for this game? First of all, materials are very important. A flat stone is much better because it increases the contact area with water. The game of jumping stones can be traced back to ancient times. Scientists used high-speed video cameras in the experiment and came to the conclusion that contact with water at 20° is the best performance (Fig. 6.10). Capillary tube. Using everyday stuff, can we prove the existence of surface tension? We can put water on a tube, and when the water level is aligned with the tube edge, we can think the tube is full of water. But by adding more water to the tube, the water Fig. 6.9 A coin can be situated on the water, not by the floating force but by surface tension of water

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Fig. 6.10 Skipping of stone on the surface of water

won’t spit out immediately and the water surface is raised in a round-flat shape. That is because the surface tension is trying to make the surface area as small as possible. Then, why is water level in tiny capillary tubes higher? We know that a rounded liquid surface has the tendency to become flat, so a sunken tendency is pulling the surface up. When the force pulling the liquid up is equal to the weight of water in the tube, the balance is achieved. Surface tension happens in the contact surface of liquid and gas. That’s because of the special condition of liquid molecules. Liquid molecules are adjacent, keeping some distance from each other. If too close, they’ll repel each other; if too far away, they’ll attract each other. The attraction force means the liquid won’t fill the entire space like gas. However, molecules near the surface are only affected by molecules below it, so the force is not balanced. Unbalanced force makes the molecules with higher speed easy to escape and become vapor, making the liquid have less molecules on the surface to arise and attract each other. So, surface tension occurs on the surface between liquid and gas. When liquid contacts solid, if the attraction force between solid and liquid molecules is greater than the attraction between liquid molecules, the liquid will

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be bound to the solid surface. This phenomenon is called adhesion. On the other hand, if the attraction between liquid molecules and solid molecules is less than the attraction between liquid molecules themselves, they repel each other. Whether adhesion occurs or not is decided by the properties of liquid and solid. For example, there’s adhesion between water and glass, but not between water and wax. No adhesion between mercurial and glass, but between mercurial and metals like copper and iron. Adhesion also relates to impurity, cleanness, and temperature. The force between individual molecules adds up as surface tension. Imagine drawing a line on the liquid surface; that line is pulled by molecules on each side. The pulling force is perpendicular to that line, and is proportional to the length of the line.

6.5 Disappearance of Buoyancy Force in Weightlessness The Earth, thanks to the buoyancy of the water, can support 10,000-ton ships and sail freely in the ocean; submarines can not only float on the surface of the water, but also like whales can sink into the water and cruise. However, astronauts demonstrated the loss of buoyancy in weightless space that the ping-pong ball did not float on the surface of the water and was submerged in the water. Why do all these happen surprisingly? So, where does the buoyancy go in microgravity, and what is the mystery of how it works? In order to understand this phenomenon, we need to recall some of the facts about buoyancy. Do you remember Archimedes’ principle? When a piece of wood is immersed in a fluid, the upward force exerted by the fluid is called buoyancy. In 245 BC, Archimedes discovered the principle of buoyancy, as described below. A body in a liquid, such as water, is subjected to two forces, one upward buoyancy and the other downward gravity, which together determine the state of the body. If the buoyancy is greater than the gravity, the object moves up and vice versa. When the two forces are in equilibrium, the object is floating relatively standstill. Let’s look at what causes buoyancy. In a gravitational field, when an object invades a liquid, the pressure under the object is greater than the pressure above it, creating an upward pressure difference, which is the source of buoyancy. So, in the final analysis, buoyancy is also caused by the Earth’s gravitational field. So what determines the magnitude of the buoyancy? The buoyancy of an object is equal to the force of gravity by which it displaces liquid. Or, to put it another way, if the average mass density of an object is less than that of water (such as ordinary wood), then it floats for a long time; otherwise it sinks (such as iron). Although the ship is made of steel and iron, it has a lot of voids in its hull, so the average mass density is lower than that of water, so it floats. However, the submarine has a ballast tank inside its body, and when it is filled with air, the submarine rises. If the air is removed and injected into the seawater, it sinks.

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Fig. 6.11 A big ship can float on the water because of the buoyant force, which is produced by the pressure difference between the water surface and bottom of ship in the Earth’s gravity

Knowing the principle of buoyancy, we can understand the phenomenon of buoyancy disappearing in space. The ping-pong ball does not float on water as it does on the ground, but stays in water, which is completely different from the phenomenon on the ground. This is because the buoyancy is generated by gravity, therefore, the weightless environment results in, buoyancy disappearance (Fig. 6.11).

Chapter 7

Viewing the Universe and Earth on a Space Station

7.1 Why are Stars not Twinkling Seen from the Space Station? On the ground, looking up at the stars in the night sky is indeed a pleasant and romantic thing, and evokes infinite reverie. There are many poets and literature to describe the brilliant starry sky and Bright Moon, depicting their heaven and the universe of fantasy and worship. Unfortunately, air pollution, light pollution, and smog make it impossible to see the stars or the Milky Way clearly. Without seeing this beautiful scene with his own eyes, the Chinese ancient poet Li Bai (701–762) would have never written the line of a famous verse “suspect the Milky Way as a river has fallen down from the Heaven.” If possible, do watch stars with your parents; it would be very enjoyable (Fig. 7.1). As described by the British poet Jane Taylor (1783–1824), “Twinkle, twinkle, little star. How I wonder what you are. Up above the world so high. Like a diamond in the sky.” Do stars really twinkle like our eyes blinking? Most people would know those stars we see in the evening are celestial objects like our sun, who gives out light on its own. Other planets, dissimilar to “stars”, do not give off light, but reflect light from other stars, like the Moon, Venus, and Mars. One may ask a question: why do stars twinkle but planets do not? Is there a burning in star and nothing on the planet? Do they twinkle seen on a space station? The reason why the stars blink when seen from the ground has nothing to do with their burning or varied luminosity. As known, the air is composed of the atmosphere, including the molecules, clouds, and dust in impurities, which are not evenly or homogeneously distributed and usually have impurities and disturbances. The disturbing and turbulent motion of the atmosphere would bend the light of the stars in different directions, causing diffraction or refraction, which makes the star light propagate through it to produce the variation of light intensity quasi-periodically, named twinkle or scintillation. So, we feel that the stars are twinkling, and in fact these twinkle phenomena are not because of their luminosity. © Springer Nature Singapore Pte Ltd. 2023 C. Zhang, An Adventure, https://doi.org/10.1007/978-981-19-9221-6_7

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Fig. 7.1 A star is observed to be twinkling on the ground, but it is not on the space station

A star is generally so far away from the Earth that it looks like a point on the ground, while a planet, like Mars, located in our solar system, is so near that it looks like a big dot seen on the Earth. The light of a point-like star is easily disturbed to form a flicker, while the planet, with its apparent size being a little bigger than that of a star, is much more resistant to interference of light and is not easy to show as a blinker. Actually, a star’s twinkle is a propagation phenomenon, affected by the atmosphere of the Earth. Some may ask that, with no atmosphere interference, would stars probably not twinkle seen from a space station? The answer is yes! This is exactly the key property of observing stars in the orbiting spacecraft of the Earth. Then, what are the other differences between watching stars from a space station and from the ground? Since there’s no air in outer space, the viewing background is completely dark except for the light emitted or reflected from other stars, and more stars could be seen, which are brighter than they look from the ground.

7.2 Space Station: 16 Times Sunrise or Sunset in One day

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7.2 Space Station: 16 Times Sunrise or Sunset in One day It is so exciting to look at the Earth from a space station, and we can get much more than just weather and geographical terrain. For example, astronauts can watch the sunrise and sunset 16 times a day. That’s because spaceships orbit the Earth at a velocity of around 7.7 km/s at the orbit height of 400 km above the ground. The perimeter length of its orbit is around 43,000 km (Earth’s radius is set as 6371 km), and it takes about 90 min for a space station to make a full orbit trip. We can calculate that in 24 h, a spaceship orbits the Earth 16 times, so the astronauts can see 16 times of sunrise and sunset. Although beautiful, it would probably make people confused, but very excited (Fig. 7.2). Watching the sunrise on a space station is not affected by the weather, rain, or thick clouds, and it is on account of the fact that no conception of ground weather is produced by the atmosphere. Viewed from the space station, sunrise is always splendid. Since the spaceships move at a very fast speed (7.7 km/s), sunrise occurs very quickly, so does the sunset. Before sunrise, the sunlight firstly makes the horizon bright. The atmosphere also acts like a lens, breaking sunlight into individual colors, while the lights in various spectrum bands of different colors can be seen. In reality, the light band or spectrum is the result of air pollution, so the most beautiful sunrise and sunset scenarios are usually seen in the most polluted areas. Aboard the spaceship, we can drink a cup of coffee, look out the ship window lazily, and enjoy a fantasy of constantly changing: the sun rises and then sets. Day and night lasted only 45 min each. It’s a wonderful and interesting paradise, isn’t it?

Fig. 7.2 On space station, astronauts can view 16 times of sunrise and sunset in one day!

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7.3 Why is Earth Shown as a Blue Planet? There are so many stars in the sky with beautiful lights and wonderful names, more colorful, amazing nebulae, and carrying the ancient legend of the ethereal Milky Way… … one of these must be your favorite. The answer may be different in everyone’s mind, or is this the wonder of the Universe? Looking at the universe from space, you ask the astronauts which is their favorite? They will answer, the most beautiful is the Blue Earth. Perhaps you do not believe that, in the universe, such as water droplets as flexible, such as clouds as pure, such as green trees full of vitality of the Earth, is indeed a most special and beautiful planet. Looking from outer space, the Earth is a shining sphere with white and blue stripes, wrapped in a light blue atmosphere. In the picture taken from outer space, we can see the Earth shown as a blue planet, so what makes the Earth color blue? About 30% of the Earth is covered by land and 70% by oceans. The oceans were joined together, and it looked as if the land were floating on the sea. Looking at the Earth from space, the astronauts see a beautiful blue planet because the ocean water appears bluish. Really, it is exciting to look at the whole planet, our home, in blue from a space view. During the daytime, the Earth is mostly light blue. The oceans account for about 71% of the 510 million square kilometers of the Earth’s surface area, or about two and a half times the land area. Although the Northern Hemisphere has about two-thirds of the world’s land, its land area is only about 39.3% of the Northern Hemisphere’s total area, and the rest of the area is about 60.7% of the sea. The oceans of the Southern Hemisphere, the Eastern Hemisphere, and the Western Hemisphere are also larger than the land area. If we use the cross point of 0 longitude and 47 latitude north and the cross point of 180 longitude and 47 latitude south as two poles, then divide the Earth into a land hemisphere and water hemisphere, although the land hemisphere has 81% of the total land, it still has less surface area than water. This shows that on any part of the Earth, there’s always a bigger ocean than land. In addition, the average depth of the ocean is about 4000 m, and the volume of water is about 13.3 billion cubic meters, accounting for 96.5% of the Earth’s total water reserves. Unlike land, all oceans are connected to each other, forming a unified vast water system that makes the continents seem to float on the ocean. The ocean itself has no color, then its color comes from the sunlight’s reflection and scattering into our eyes. As known, a ray of sunlight contains a spectrum of seven colors, red, orange, yellow, green, blue, indigo, and purple, ranging from long wavelength to short one. The ocean acts like a filter and bounces off the shorter wavelengths, and the water molecules absorb the red part of the spectrum, and leave the colors in the blue spectrum reflected back into our eyes (Fig. 7.3). Since the Earth’s oceans are so vast, continuous, and exhibiting a color of blue, the Earth became a blue planet when looking from outer space. Looking from space during the day, most areas of the Earth are full of light blue, however, the Sahara Desert is dark brown. And we can clearly see, the Himalayan mountain, distinguished forests, lakes, and plains, the city lights twinkle everywhere at night (Fig. 7.4).

7.3 Why is Earth Shown as a Blue Planet?

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Fig. 7.3 From space station, Earth looks like a blue planet. Image credit NASA

Fig. 7.4 Sunrise from space station. Image credit NASA

Sunrise and sunset on a space station. What’s the difference between watching the sunrise and sunset from the ground and from space? It does have something to do with the direction of the spacecraft, but spacecraft generally don’t go west, because when they are launched, they usually go east in the direction of the Earth’s rotation to save energy. The space station moves faster than the Earth rotates, the sun rises and sets in less time from space view, and there is no atmospheric blocking effect that results in a large and red sun. When we see the sunrise from space, we don’t see the sun pop out for a moment; when we watch the sunset in space, we see the sun’s white light and its exact location on the horizon. At sunrise and sunset, when the sun

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shines on the far horizon of the Earth, it looks like a colorful ribbon, hidden in the dark background of the Earth, holding up a gorgeous gem. Why is the background of outer sky dark? On Earth, the daytime sky is blue and the nighttime sky is black, because during the day, light from the sun hits molecules in the Earth’s atmosphere and scatters them in all directions, different colored photons scatter differently, which results in a color of blue. At night, when the Earth is turned away from the sun, the sky looks black because there is no bright source of light like the sun to scatter. However, if you were on a space station, without an atmosphere around it, like on the moon, outer space would be black, since there is no scattering of air. Whether in daytime or at night, the backgrounds of the outer space pictures taken on the space station would be black. Viewing the auroras from outer space is so exciting. As known, auroras are a colorful light-emitting phenomenon, occurring over regions near the Earth’s magnetic poles, which are bright and beautiful at night sky near the Earth’s North and South Poles, which originate from the sun’s outbursts throw streams of charged particles (solar wind) into the Earth’s magnetic field. In terminology, it is called the aurora Australis at the South Pole and the aurora borealis at the North Pole. These high-energy charged particles (solar wind) collide with the Earth’s atmosphere, and excite (or ionize) molecules or atoms there to give off the light. From outer space, astronauts viewed and photographed auroras, a spectacular display of green light in the Earth’s surface atmosphere. Also, when a faster particle flow of solar wind meets a slower one on the North Pole, energy is transferred between the two, creating even more intense luminous lights, named an “Interplanetary shock”, which has been observed by astronauts. Strategy of viewing Earth: selection of the space station orbit. The International Space Station (ISS) has an orbital inclination of 51.6° and travels from west to east at an altitude of about 400 km, taking approximately 90 min for each run. Why does it not select the orbit around the equator? At an altitude of 400 km, it is impossible for an astronaut to have a clear view of the entire Earth’s surface, and most of the major sites of human activity, e.g., cities and farms, are not near the equator. If ISS selects an orbital tilt of about 45°, the space station scans a small part of the Earth by each orbit, and it gradually sees most of the Earth’s areas as the ISS orbital plane moves. According to the ISS height, the exact number of one complete orbit around the Earth per day is usually less than 16 (15.5–15.9/day). After each orbit, the spacecraft shifted to the west for about 22.5° of longitude, as measured by the position of the orbit across the equator, because the Earth rotates to the east by 22.5° in 1.5 h (90 min). Moreover, during this time, one part of the Earth is seen at night, and the other part is seen during the day. The orbital altitude of ISS decreases over time due to Earth’s gravity and atmospheric friction drag, and its orbit is regularly restarted and changes slightly. As a comparison, the Chinese space station (CSS) operates at an inclination of 41° to 42° and an orbital altitude of 340–450 km, and its orbital parameters are similar to those of ISS (Fig. 7.5).

7.4 Moon is Much Brighter Seen from the Space Station

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Fig. 7.5 Photo of the Chinese space station. Image credit China news agency

7.4 Moon is Much Brighter Seen from the Space Station On the ground, we can view many colors of the moon, which depends on the atmospheric circumstance, weather conditions, viewing angle with the moon’s sight of line, etc. The moon does not shine, like a mirror, reflecting the sun’s light to the Earth. Through the absorption and scattering of the Earth’s atmosphere, the moon will reflect the sun’s light to the ground. When the night sky is clear, the seven main color components are not blocked, thus it reflects the silver-white light, by passing through the atmosphere and meeting less dust and small water droplets. When the weather and air conditions are not so good, the reflected light passes through the atmosphere and encounters more blocking elements, violet and blue light are blocked more, and the rest of the light is mixed together to form a yellow color. In space, watching the moon is more fun. In the daytime, the moon is a little bit light blue in a dark background, which should be influenced by the color of the Earth’s ocean and sky. During the night, the moon is much brighter than what we see on the ground, as if the moon is giving off light on its own, because there is no interference and blocking of the Earth’s atmosphere and clouds at all. Why do these happen? Sunlight is composed of a spectrum with different wavelengths, and dust particles in the Earth’s atmosphere can reflect blue light better than other wavelengths, and make the sky seem blue to our eyes. Dust particles and fog droplets diffuse lights in different directions. Tiny particles like molecules and atoms can also diffuse lights, whose intensity of diffusion is inversely related to the wavelength. And the red light’s wavelength is twice of blue light, so the diffuse intensity of blue/purple light is nearly ten times that of red light. Since the human eye can feel blue light better than purple light, we see the sky as a color blue (Fig. 7.6).

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Fig. 7.6 View of moon rise from space station. Image credit NASA

7.5 Can the Naked Eye See the Great Wall from the Space Station? The students often ask the question, by means of the naked eye can we see the manmade wonders, e.g., the Great Wall of China and the Egyptian pyramids from space, in the low Earth orbit (350–500 km above the ground)? The answer is most probably no. It is true that there are many man-made structures that can be easily identified from space by viewing through instruments and telescopes, including highways, airports, dams, cities, wheat fields, bridges, even the human body, and so on. Looking at the Great Wall from 300 km away is like looking at a hair from 30 m away. The human eye has limited resolution and cannot see the air clearly. In the moving space station at a 300 km orbit of the Earth, there is no chance to see the Great Wall. Moreover, the Great Wall is made of the same material around it, making it very hard to distinguish from space (Fig. 7.7). You must employ the telescope in order to have a look at the Great Wall, and you also should aim in the right direction where the Great Wall is located. Of course, with a telescope, we can get a clear shot of even our family. Moreover, if you want to see the Great Wall from space above China, the weather condition should be very good, and the orbit motion velocity 7.7 km/s, which needs the camera shot quickly. Actually, after the first Chinese astronaut returned from a space station, he was asked by a journalist, “Have you seen the Great Wall?” “No”, answered the astronaut without hesitation. It was a rumor that came out of nowhere, and that the “Chinese Great Wall is the only man-made structure that can be seen from space on Earth.”

7.6 Space Telescope from Hubble to James Webb

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Fig. 7.7 One can see the Great Wall by the telescope but not by the naked eye

Theoretically, it is absolutely impossible to see the Great Wall from space. Because the wall is narrow and irregular, it is difficult to see irregularities from space; moreover, the average width of the wall is less than 10 m, making it easy for the wall to be obscured by the surrounding terrain and blended into the background. Thus, at a height of 20 km above the ground, it’s very difficult to tell it apart the Great Wall, which is completely out of sight at a height of 60 km. Well, the space station orbits 300-500 km from the Earth, making it impossible to distinguish and watch the Great Wall with the naked eye. Moreover, to see the Great Wall from the moon is like looking at a hair from 2,000 m away. Even if the sky is clear, it is obviously impossible. The above facts and explanations may disappoint people, but we have corrected the common misconception with the facts. No doubt, saying “astronauts have seen the Great Wall from space” is a total rumor, due to a lack of basic common sense (Fig. 7.8).

7.6 Space Telescope from Hubble to James Webb There exist lots of planets, stars, galaxies, pulsars, black holes, unknown celestial bodies, and even extraterrestrial civilizations in the vast universe. Today, it is difficult to make astronomical observations near cities because of light pollution. To avoid light pollution, the observatory is built in remote locations. But even there, we cannot get rid of interference—the atmosphere.

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Fig. 7.8 On space station, one has to employ a special telescope to view the summit of Mount Everest

There are some reasons why space telescopes should be launched out of the atmosphere. First, if you look at the stars, your observations are influenced slightly, because atmospheric turbulence deflects light in different directions, blurring clear images and making it impossible to study stars and galaxies in detail. Second, the atmosphere not only protects us from intense harmful radiation, X-rays and ultraviolet rays, but it also prevents us from observing them. The atmosphere allows most of the visible light to pass through, but it still deprives us of 20% of the light that is important for our study of a very distant universe. Third, infrared observations from the Earth are very difficult because all objects and we give off fairly strong infrared light due to a certain temperature. But outer space is a cold place, and our observations from there will not be disturbed by this type of pollution. Targets of the Webb telescope include the explorations of the black hole, birth of stars, galaxy, big bang, exo-planet, and so on. Webb is a powerful astronomical instrument ever made, with a primary mirror of 6.5 m in diameter, providing a significantly larger collecting area than that of the space telescope Hubble which has a mirror of 2.4 m in diameter. As expected, Webb should have lots of discoveries about the secrets of the universe (Fig. 7.9).

7.6 Space Telescope from Hubble to James Webb

Fig. 7.9 Image of James Webb space telescope. Credit NASA

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Chapter 8

Wonderful Space Life: Part One

Throughout human history, tales of extraterrestrials and unidentified flying objects (UFO) have circulated widely in the world, and children have grown up with legends of Little Green Men (LGM) saving the planet. Do they exist? How do we find them in the universe? Are they looking for our planet, too? How do they live in the weightlessness of outer space? How is their planetary environment different from Earth’s? This series of questions has been around the students, causing them to frequently propose lots of curious questions. The United States, Russia, the European Union, and China, together with a series of countries, have put into the construction of space stations, and mankind is gradually uncovering the mysteries of outer space. To answer those questions about space, students can first explore how astronauts live and work around space life. Outer space is defined as a particular space region that is more than 100 km above the Earth’s surface. The space station is located approximately 300–500 km from the ground, where the space environment is completely different from that of the Earth, where the free-falling orbit motion results in weightlessness. So, as a comparison, on the space station and the ground, the life of astronauts is totally different, including a series of daily life elements and actions like clothes, food, living, walking, eating, drinking, bathing, sleeping, fitness, and so on. Let the astronauts lead us to explore the outer space life and work in detail, to understand their strange experiences.

8.1 Playing with Fire on the Space Station: Round Flame Have you heard that astronauts even play with fire on a space station? Their real goal, of course, is to study the properties of flames in weightlessness. What does a flame look like? It is a dazzling burn, and the candle’s tear-shaped flame was caused by a rising stream of hot air. If the air flows smoothly around the candle flame, it looks like a yellow ribbon dancing in the breeze, a blue cone. Either way, the shape of the flame depends on gravity, especially the fact that hot air is less © Springer Nature Singapore Pte Ltd. 2023 C. Zhang, An Adventure, https://doi.org/10.1007/978-981-19-9221-6_8

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dense than cold air, and therefore it rises. In weightlessness, this “convection” effect no longer works, and the flame is more spherical in shape. On the ground, we lit a match and then set off fireworks, which is the fun of children’s childhood. After a while, we soon saw that the match’s flame showed like colorful flowers, rising slowly, and the breeze gently swayed the flame as it danced, which brought warmth, light, and heat to the winter night. These are wonderful childhood memories. So why is the fire flame dancing up instead of burning down? Furthermore, how do flames move in the circumstance of weightlessness on a space station? Firstly, combustion in air is a kind of exothermic and luminescent chemical reaction, the process of which may be complex, and light and heat are the fundamental physical phenomena occurring in the combustion process. The occurrence and development of the material combustion process must require the following three necessary conditions, namely combustible (burnt object), oxidant (air or oxygen), and ignition source (lighters or matches). Generally speaking, the flame appears as three layers, with different colors, and its temperature rises from inside to outside. Therefore, the outermost flame is the most dangerous; children must be careful not to be burned and scalded by it. How does the shape of a flame come about? On the ground, the flames are tearshaped or pear-shaped. As we all know, hot air rises in a gravitational environment, and the air around the flame becomes less dense as it is heated. If the flame wants to continue burning, it will rise with the air, so it will form a teardrop shape. Of course, if you blow air toward the flame, or if the breeze around you disturbs the air, the shape of the flame will also be affected. Can fuel still burn to create a flame in a vacuum or air-tight environment? The answer is no. Some students may ask: Why does the sun burn? This is because the sun’s “burning” is different from the burning normally seen on the ground. It is actually a form of thermonuclear fusion coming from inside the sun. The high temperature and pressure of the sun’s interior ignite the hydrogen nuclei to combine and form helium nuclei, simultaneously releasing, a lot of light and heat, the process of which is different from that we normally see on the ground. On the space station, astronauts conducted combustion experiments and found that the flame shape was no longer a teardrop, but a round ball of fire. Why is that? The mystery of the shape of the “fireball” is immediately known, according to the reason for the formation of the fire we mentioned above on Earth. Yes, in the microgravity or weightlessness of space, hot air does not rise in any direction, so the flame exhibits a spherical shape. Even more surprisingly, flames can also occur in space, where the oxygen content and the ignition temperature are lower than those on Earth. As expected, many mysteries about the chemical and physical mechanisms of space combustion need to be solved, and we are required to continue to study as scientists in the future (Fig. 8.1). Spacecraft Flame Experiment: NASA launched a cargo spacecraft (Cygnus) to the International Space Station Space Station (ISS); returning from the mission, NASA used a remote control to set the spacecraft on fire before burning Cygnus in air. The purpose of the spacecraft Fire Experiment is to study the spread of fire in the

8.1 Playing with Fire on the Space Station: Round Flame

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Fig. 8.1 The shapes of flames on the space (upper panel) and ground (lower panel) are different, spherical and pear-like, respectively

microgravity environment of space, and spacecraft fire is one of the biggest concerns in space exploration. In this experiment, half a square meter of material, a special fabric, is burned safely without additional risk to the astronauts on the space station. A detailed understanding of the spread of fire in the space environment will help develop better fire-resistant materials and technologies, thereby improving the safety of space travel, which is essential for the safety of future astronauts. With plans to build the bases on other planets like the moon and mars, we need to minimize fire risk, so we have to study how fire burns in space and other different gravity circumstances. The experiment will help NASA choose fire-retardant materials and designs for space suits, capsules, and habitats, and help determine fire-fighting strategies in space. On Earth, gravity has a profound effect on flames, but in space they can behave unexpectedly, making them more dangerous. Such data could never be collected on Earth, but mathematical models could be used to predict how the material would burn on the moon, mars, or other environments.

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8.2 Will People Shed Tears on the Space Station? When children fall down, the pain makes them cry and shed tears. This is due to the influence of gravity, tears will “fall” down. But the space station is weightless, and tears do not flow down, nor do they flow. Astronauts often say, “In that moment, I shed tears.“ We should know that astronauts do not have great emotional control problems, but it is not easy to shed tears in the absence of gravity. So, when they feel “tears”, they will use a towel or paper tissue to wipe away the tears, because the tears in the space capsule will not only endanger the safety of astronauts, but will also be more likely to cause an accident (Fig. 8.2). But the question is, in the zero-gravity condition, can the tears be formed? As known, the lacrimal gland is a small almond-shaped type situated in the upper part of the eye socket, near the position of your eyebrow, which produces the watery tears. The tear gland creates a thin layer of water in front of our eyes, keeping the eyes moist, and they are the source of tears. Whether in a huge ocean or in a cup of coffee, the shape of the liquid depends on the container boundary because of the Earth’s gravity. The surface tension in the liquid causes the molecules on the surface Fig. 8.2 Because of the weightlessness on the space station, the tears of astronauts cannot flow out smoothly, and stick to the eyes or stay at the corners of the eyes (upper panel), as a comparison to that on the ground (lower panel)

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to become closer to each other. On the space station, if we want to pull down the tear liquid, it will come together to form the smallest possible shape, the spherical liquid ball.

8.3 Will the Astronauts Become Taller in Space? Do you expect children to grow taller quickly, and then suggest they fly into the space station? Let us see the possibility. When we are on the ground, the spine is stressed by the gravitation-induced weight, but in a weightless circumstance, the spine is no longer affected by gravity, and it is stretched, which causes the body to grow by 2–3 cm. But don’t get too excited by this news. However, once back on the ground, the astronauts will recover to their original heights. Furthermore, if astronauts stay in space for five to ten years, will their heights change? The answer is no. Staying in space for too long can lead to bone loss and muscle atrophy, which can damage the astronauts’ health. Therefore, if you expect your kids to grow taller, they should rely on regular exercise and a balanced nutrition diet, not think of space travel (Fig. 8.3). Likewise, because of weightlessness on the space station, the astronauts appear to be getting fatter. Can the children think about the reason for this problem by the previous analysis? In fact, under weightless conditions, human blood and body fluids will be redistributed in the body. On the ground, due to the influence of gravity, the blood will be concentrated in the lower part of the body, and in space, blood and body fluids become redistributed on the whole body, so the faces of astronauts look very full or fat. However, are the astronauts really getting fat? Of course not, because of the weightlessness, the loss of muscle mass, and the loss of body fluids, it is a fact that most astronauts will lose weight at 3–4% after one space trip. Now, do you get the right answer? You might wonder: if the space station diet hadn’t made the astronauts fat, would it have been morbidly bloated? In fact, this fatting phenomenon is a normal space physiological reaction. Our body water content is very high, at more than 60%, and there exist in the flow of water in the blood and the cells of supporting a variety of chemical reactions. In the weightless condition, we certainly have more body fluids in the head than on Earth, so the astronaut’s face looks fatter (Fig. 8.4).

8.4 Spacewalking Outside of a Spacecraft A spacewalk is the activity of an astronaut’s getting out of a space station vehicle, which is also named an EVA that represents extravehicular activity, or EVA usually means spacewalk, while the astronauts go out of the space station to fix the equipment or do experiments. In reality, an astronaut walking in the vast universe circumstances is an essential process of managing to accomplish many kinds of tasks in the extravehicular environment, such as on the moon and the planet. Spacewalk is a

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Fig. 8.3 The body height of astronaut on space (right panel) is higher than that on the ground (left panel)

key technology for manned space flight, which is an important way to install large equipment in orbit, conduct scientific experiments, put satellites, and inspect and repair spacecraft. Walking in space is much more difficult than people walking on the ground, and getting the job well done in the spacewalking requires a lot of special technical support (Fig. 8.5). A spacewalk is not only an acrobatic performance. The Manned spacecraft, especially the maintenance and assembly facilities of the orbiting space station, deck space payload recovery, and care, all require astronauts to walk in space in order to accomplish extravehicular operations effectively. With the expansion of human spaceflight activities and the increase in space flight frequency, the number and distance of spacewalks are gradually increasing. The first-ever spacewalk of human was conducted by Soviet cosmonaut Alexey Leonov on March 18, 1965, who walked five meters away from the capsule and whose EVA lasted only 12 min and nine seconds, which has become one of the most

8.4 Spacewalking Outside of a Spacecraft

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Fig. 8.4 The faces of astronauts are fattened on space station. Credit China news agency

Fig. 8.5 Alexei Leonov (1934–2019), a Soviet cosmonaut who conducted the first spacewalk in history in 1965. Credit TASS

glorious moments in the history of space exploration. Now, astronauts from the US and Russia can proceed with the spacewalks outside the International Space Station lasting as long as over eight hours. In addition, the American and Russian astronauts on the International Space Station have completed more than 100 spacewalks, while the Chinese space station has only just begun to follow suit, and has also completed several extravehicular activities and spacewalks. With the development of space technology and the improvement of people’s understanding of the space environment, mankind will gradually solve some difficult problems in space movement, and the dream of flying in space will become true in the future (Fig. 8.6).

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Fig. 8.6 The crew of the space station goes out together by spacewalk for the conventional maintenance and repair jobs, while the safety tethers need to be fastened

Chapter 9

Wonderful Space Life: Part Two

9.1 Sleeping in Weightlessness Children might say, “space station is a wonderful world. There are a lot of strange things that happen in weightlessness or microgravity condition.” Yes, in weightlessness, the astronaut’s body relaxes completely, forming an incredible posture. Space experts say that astronauts’ living and working in the space station is much like a circus show, including the magic, acrobatics, and fantasy jumps, arousing a wave of cheer and admiration. Often, miracles beyond imagination do happen before one’s eyes. Can you sleep in a standing up or handstand posture? Since the astronauts lie in the weightless circumstance, the feeling of null gravity can cause them not to distinguish the direction there, therefore the sleeping posture can be of any type without limit, such as standing, sitting, lying down, and even handstand! The best advantage of sleeping in the space station is that you don’t need a bed. You just slip into your sleeping bag, zip it up, stick your head out, put earplugs in your ears, put on goggles, and you’re ready to go to sleep. Of course, you can find a convenient place to sleep without disturbing the movements and without distractions from others. The space station has a special sleeping compartment for each astronaut. During sleep, arms should be placed in front of the chest and secured against the body to prevent the astronaut from floating in the cabin due to the thrust generated by exhalation or rollover. The implementation of these sleep safety measures can avoid injury to themselves, contact with instruments, and damage to these space equipment. How to prevent sleepwalking in space? Since astronauts can sleep in space like sailboats, do children worry that while they sleep, they unconsciously dream to fly off into space? Space experts have an answer: the astronauts have their own bedroom, but there is no bed there. To prevent sleepwalking and drifting away, the astronauts will attach sleeping bags to the walls of the capsule. On the one hand, they can make sure that the capsule doesn’t collide with the body and they don’t drift away. On the © Springer Nature Singapore Pte Ltd. 2023 C. Zhang, An Adventure, https://doi.org/10.1007/978-981-19-9221-6_9

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Fig. 9.1 For a safe sleep, the astronauts need to anchor themselves to the spacecraft

other hand, this position is as comfortable as sleeping in a bed on the ground, while you can turn over at will (Fig. 9.1). Why do astronauts sleep on spacecraft wearing earplugs and goggles? When we are on a long flight trip, the stewardess will give us earplugs and goggles to avoid interference around. However, the astronauts sleep in space with not so many crew members around. Why do they need earplugs and goggles? Shall we consider the reasons? The length of the Earth’s day and night is caused by the Earth’s rotation and is also related to the seasons and geographic latitudes. At the equinox, the days and nights are the same lengths. Day and night on the ground produce the temporal concept of human activity that affects human physiology and living habits. The space station’s concept of day and night is different from that on Earth. In orbit, the part of the space station where the sun shines is its day, and the part in the Earth’s shadow is its night. It orbits the Earth in about 90 min, so the sun rises and sets once a day on Earth and about 16 times a day on the space station. However, the length of day and night on a space station is not simply 45 min each. But the day–night view from the space station is also different from the view from Earth. There is no scattering of atmosphere, water vapor, or dust particles in outer space, so you can see a bright ball of light while facing the sun and a dark background while facing away from it. If the space station keeps looking back at the Earth, the length of the day and night is inconsistent because of the scattering, refraction, diffraction, and light pollution from dust particles in the atmosphere, and the daytime is slightly longer than the night. Such rapid day and night shifts can disrupt the astronauts’ internal clocks. On the Earth, people’s custom is “the daytime work and night rest”, but the day–night cycle on the space station interferes with the astronaut’s normal life order, and affects their normal rest habits, therefore their

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Fig. 9.2 For safe sleep, the astronaut is reminded to wear earplugs and goggles on spacecraft

sleep needs the earplugs and the goggles, which isolate the disturbance caused by the extreme variations of environments. One astronaut’s diary described the quick change of day-to-night on the space station as follows: “In the morning, the computer-controlled clock woke us up. We opened the curtains and looked at the space outside. The sun was shining, and the sky was beautiful. But a little while later, the sunlight was gone, and the sky was gray and dark, while the night came. Should we go to bed again?” (Fig. 9.2). Unusual sleep feelings of the astronaut in weightlessness. In weightlessness for a long time, the human body is in a highly relaxed state, and the psychological sensation occurs abnormally. Sometimes, people hallucinate and feel as if their head and limbs are separated. Space experts have studied a similar case in which an astronaut felt his arms float around like a monster in his sleep, and found that, on Earth, we have an empirical understanding of what happens to parts of the body under the action of gravity, but the weightless environment undermines the experience we have, and the sudden relaxation of freedom deprives the brain of its ability to judge the relative position of limbs. So it’s not surprising that astronauts occasionally experience hallucinations when they feel sleepy.

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9.2 Drinking and Eating on a Space Station The fun of drinking. “Water flows downward” is the truth of the Earth, and this is due to the influence of gravity, which drives fluid from a position of high gravitational potential energy to a place of low potential energy, so water flows from high to low altitude. In weightless environments, however, a glass full of water, facing up or down toward the Earth, does not flow out, because the gravitational potential energy of the space station is the same everywhere. So, on the space station, if the astronaut wants to drink water, it takes a little skill! Generally speaking, space drinking water is sealed in a bag, the astronaut can spray the water into their mouth, or use a straw to slowly suck water. The clever astronauts showed us a great way to drink water by first squeezing the water balloons out of the water bags, letting them float in the air, and then eating them. Interestingly, Chinese astronauts developed the trick of using chopsticks to pick up water ball and put them in their mouths. This is because the surface tension of the water causes water molecules to get together to form a small water ball that floats in space. To prevent accidental spills from drinking water, the astronauts could jam the straw, stopping the flow of water so it wouldn’t leak out. When a large amount of water leaks into the space station, the water droplets will float in the air, which will affect the safety of many surrounding electronic equipment (Fig. 9.3).

Fig. 9.3 Eating a water ball with chopsticks on space station

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Eating in weightlessness. Considering the particularity of weightlessness, space food has very special requirements, for example, avoiding powdered food, having no bone core and other superfluous garbage, and being convenient to eat and rich in nutrition. If food is full of solid debris, it can become too dangerous to get into an astronaut’s respiratory tract or into a gap in equipment if it is chewed up and blown around, drifting everywhere in weightlessness. Experts suggest that a certain viscosity in the soup is good for space food, which can ensure that food debris does not appear everywhere and fly away; this is because the motion of liquids in weightlessness is mainly controlled by surface tension, cohesion, and adhesion, while, on the ground, it is mainly controlled by the gravity of the Earth. Early space foods were filled with paste, such as fruit and meat dishes mixed in paste, packaged in plastic bags, like toothpaste, and they are hand-squeezed into the mouth. The astronauts, however, said it did not taste good. Now, space food has been greatly improved, with a huge variety, of nearly 100 foods and beverages on the menu, including ham, sage, juicy beef, chocolate pudding, fruit, cake, juice, coffee, etc. The food on the Chinese space station is even more abundant. In addition to the traditional Western food variety, there are also Chinese dishes such as rice, stir-fry, seafood, and Chinese tea. What if an astronaut gets hungry in space? The space station’s kitchen has a hotair heater that can cook meals in 30 min, and can be used to make homemade yogurt and soy milk. Also, the space kitchen has a water dispenser that“forces” the water to take turns and reheats it instantly, so astronauts can drink a glass of water at any time. Bread or rice cannot go out of the container freely, because there is no effective gravity to help on a space station. The food could float and fly in the air, so the astronauts would have to find new ways and means of eating. First, fix all kinds of food and covers and open the food bags carefully, take out the fork or chopsticks, and finally take food to your mouth. Generally speaking, food will be packaged in a small piece so that it can be easy to get into the mouth (Fig. 9.4). Of course, it is also necessary to heat the food. So can kids guess how the astronauts heated the food? With Fire? No, it’s dangerous! Astronauts use microwave heaters to bake food in space kitchens. To keep the heated food from floating around, it is necessary to secure it and then turn on the power. After heating, how to eat it? Tables and chairs are indispensable. The space table is magnetic and attracts metal cutlery. At dinner time, the astronauts need to put their feet on the floor, face the table, and then start eating slowly. Tableware on the space station: chopsticks or fork. On Earth, our ancestors left behind many useful tableware such as spoons, forks, and chopsticks from their past life practices. A spoon is an effective tool for liquid food. Forks and chopsticks are generally used in solid foods. They cannot be used in liquid or liquid foods. These are our daily experiences. With spoons and forks, or spoons and chopsticks, we can finish a meal smoothly. So, what’s the best cutlery in space? Milk and water do not flow in weightlessness because they are subject to viscous forces. Instead, they float in the

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Fig. 9.4 To have a birthday party on space station, we must be careful that all foods will float in space but are not fixed on a table like the situation on the ground

air in a spherical shape. Astronauts can not only use a spoon to drink water, but also use chopsticks to hold water balls, and put them in their mouths to drink. Therefore, in weightlessness, chopsticks are a useful tool for both eating and drinking. We are afraid that the Chinese ancestors didn’t think of this when they invented chopsticks.

9.3 Brushing Teeth and Taking Shower on a Space Station Brushing teeth on a space station. In daily life, we use water and toothpaste to solve hygiene problems, but in weightlessness, these two things, like a naughty brother, do not favor the flow but prefer a farce. So what exactly are astronauts doing to clean their teeth? Can space toothpaste be edible? So, the astronaut toothpaste came out. This is a kind of toothpaste that does not produce bubbles and can be absorbed by the body. As for the toothbrush, astronauts can use special fingerprints instead of toothbrushes, or can also take their favorite toothbrush off. Many astronauts carry personal hygiene boxes, which are equipped with toothbrushes and special toothpaste. When brushing the teeth, fasten the water bag with a rubber band first, take out the toothbrush, then attach it to a fabric bag, and next get the toothpaste. Use a finger to gently flick the straw clip of the drinking bag, so that the water can penetrate into the toothbrush. And then squeeze the toothpaste on the toothbrush. Now you can brush your teeth. Finally, rinse your mouth and place your toothbrush on a towel to remove the moisture, and put the toothpaste in the box. It

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may sound very simple, but the astronauts need to practice hundreds of times to be able to swallow the toothpaste, which is not easy to try. If the space travel time is short, can you not brush your teeth? Astronauts told us that not brushing your teeth is very unhygienic behavior. Even under weightless condition, bacteria can thrive; please don’t hold fluky psychology, because they will attach to and even into the stomach in the oral cavity to affect the body’s health, which is a kind of dangerous behavior. Astronauts can also use a special gum to maintain oral hygiene, which is a strong chewing gum. After astronauts chew it, the bacteria can be adsorbed down to achieve the purpose of cleaning teeth. How to take a shower on a space station? Personal hygiene is a headache for astronauts working in weightlessness. Under the Earth’s gravity, the bath water flows from top to bottom, so if only we turn on the sprinkler, we’ll be able to shower, simply and conveniently. But, in weightlessness, all these become difficult problems. Water flows in the air under gravity, but does not flow in weightlessness. So, scientists came up with an idea for the astronauts to use a water gun to spray water, which would produce a tough problem if they do not treat with dirty water. Aerospace engineers designed a closed bathroom. Let the astronaut’s body inside and the head exposed outside to prevent the water mist from going into the respiratory tract, or there exists the risk of choking water. When you take a shower, you should also pump out the wastewater and use the water purification device to reuse the recycled water. So, taking shower in space is a complicated and laborious job. Because of its limited payload, water storage is invaluable. So, the astronauts usually just wipe their bodies with a bath sponge every day, and taking shower is only a weekly luxury welfare.

9.4 Space Toilet and Water Cycling System Defecating and urinating are the daily physiological needs of astronauts, and going to the toilet on the space station is a daily necessity. However, using a toilet in weightlessness and disposing of waste can be tricky, so how do astronauts do it? Let us take a look back at the history of space exploration and learn about the relaxing and exasperating moments astronauts spent in toilets. In the early times, it was very simple for astronauts to go to the toilet in outer space. All they had to do was put a plastic bag around their buttocks, and then the astronauts directly excreted feces, immediately sealing up the feces in the plastic bag, which prevents the excrement from floating in the air. Later, space engineers invented the simple adult diaper, but cleaning them up in space was a laborious task. Now, ingenious devices were used to help astronauts defecate in space, and toilet systems were invented to convert urine into drinking water. However, such space station toilets are expensive to build, costing up to $20 million (or Euros). Today, space stations are equipped with fully automated toilets, where astronauts can sit and relax, enjoy music, and complete the necessary tasks of the day.

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In order to prevent feces from floating in the capsule, the space station toilet is designed with a small aperture of about 10 cm, which generates negative pressure, used to absorb residues. Both the International Space Station and the Chinese space station are equipped with facilities to collect water-based liquids, including urine and water, to create natural water. Similar to the water cycle process on Earth, urine is recycled and used as drinking water for astronauts. In fact, the urine after the space station treatment is as good as pure water, and the human body will not have any adverse effects drinking it. Next, the students put forward a new question on how to deal with the astronauts’ defecation? Very simple! An absorber in a space toilet stores the waste, packages, and brings it back to Earth for further research study (Fig. 9.5). Fig. 9.5 A new space toilet by NASA, named the Universal Waste Management System (UWMS), is installed on the International Space Station. Credit NASA

Chapter 10

Extended Knowledge and Products Based on Space Exploration

10.1 Effects of Weightlessness on Women and Men It is reported that more than 60 women astronauts, while the number of male astronauts of about 500, have flown in space, so the gender ratio of women to men is only 12%, since the first Russia woman, Valentina Tereshkova (Vostok 6), flown in space on June 16, 1963. In weightlessness, the relationship between gender and health is a topic worth paying attention to, and its influence on spaceflight brings a new element. Whether or not space exploration treats men and women equally, after all, there are physical and psychological differences between both, as well as differences in physical fitness. Compared with men, women astronauts have to overcome more difficulties in space, but they also have their unique advantages. A study found that, the female astronauts are more likely to feel dizzy and nauseous while going into space, while males are more likely to feel nauseous and nauseous on their way back to Earth; while back on Earth, men have more visual and auditory problems, and women don’t have this problem but have problems with their blood pressure, making them dizzy. It is not clear whether the subtle differences between the two genders are due to the hormonal differences or physiological changes. Will space travel affect fertility? There is no clear evidence that female astronauts’ fertility is affected. In fact, there are many examples of male and female astronauts having children after returning to Earth from space flight. As known, astronauts are at risk of radiation exposure in space, but how it affects female fertility is unknown. After returning to Earth from space, the male astronauts’ physiological ability will return to normal after a short period of exhaustion. So there is no sign of long-term fertility effects from space travel. In the early days, when scientists selected astronauts, they considered that women had a number of advantages, such as being lighter and smaller, meaning that a lot of money could be saved on the launch. In the end, however, the scientists chose male astronauts, and worried about the physical strength of the women, and also © Springer Nature Singapore Pte Ltd. 2023 C. Zhang, An Adventure, https://doi.org/10.1007/978-981-19-9221-6_10

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Fig. 10.1 Valentina Tereshkova (1937–), a Russian astronaut from the Soviet Union, is the world’s first female astronaut who orbited the Earth in 1965. Credit TASS

concerned about menstruation in space. The toilets on the space station are specifically designed to deal with the menstrual cycle of female astronauts, and diapers are used to dispose of waste. In addition, scientists have invented space-specific sanitary napkins designed for female astronauts to use during their periods, and have proposed an effective method of using drugs to delay the time of menstruation (Fig. 10.1). When it comes to space walking, women are physically weaker than men. Astronauts in space not only need to walk, but to complete a variety of space maintenance, equipment installation and scientific experiments, these works need the powerful physical strength. Men and women have their own strengths in cardiovascular function. Endurance test found that the female astronauts prone to syncope, heart rate increased and blood pressure decreased significantly. Psychologically, women are better than men in space. In terms of physical structure and psychological quality, women’s endurance is stronger than men’s, and the stability of psychological quality is higher than that of men’s. Looking to the future, human exploration of space is endless. If there are no female astronauts, how to carry on the family and how to settle down in the other planets?

10.2 Animals Respond to Weightlessness In early times, Russia and US started their experiments of sending the animals into the Earth’s orbit. Laika, a Moscow street dog, is the first animal astronaut in space, but unfortunately she died there. In May 1959, NASA sent the two monkeys Able (rhesus monkey) and Miss Baker (squirrel monkey) into space and they successfully returned back to Earth. Why do scientists study the life of animals in space? While scientists may not really care about how animals such as mice respond to the cosmic environment, animal data can be transferred to human models to help us prevent or solve the problems we face today. Animals have access to space for scientific research only when it is absolutely necessary. In fact, researchers tend to work with computer models to perform the simulations or directly with astronauts. For some experiments, however, animal

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experiments are more appropriate. For example, experiments that closely control the diet of their subjects, which would be a burden on human astronauts who are generally unwilling to eat the same amount or type of food every day, which would be much more convenient for animals. Scientists believe that the future of manned deep space flight must use animals before space experiments, because after all, we cannot use the human body to study the impact of various factors in space flight. Moreover, human beings and animals have been living on the Earth for a long time. Their body structure and physiological function are adapted to the gravity environment from the view of social and individual development. When humans and animals enter space in manned spaceships, the weightlessness will have obvious effects on the body and lead to a series of physiological reactions. By studying the different reactions of humans and animals, scientists can gain insight into the resilience of different species to space travel, understand the evolution of their organs, and accumulate more knowledge and information for future extrasolar or exoplanet migrations. For example, in the future on the surface of Mars we establish a human settlement base, where the surface gravity is less than half of the Earth, called as a low-gravity environment. At the time, scientists were considering which animals were better adapted to life in Mars? They will be the candidate pioneers of human experimentation, the first to colonize exoplanets. Do animals like living in microgravity? Does the transition from walking, crawling, and flying to floating mess up their physiology or nervous system? Most animals appear to behave in three states under microgravity, showing a lovely, pity, and unfortunately embarrassing situation. Some of the animals seemed to be completely stunned, seemed to feel strange, or struggled with no idea what to do. Others are more extreme, plunging into chaos and frantically waving their bodies. A few seem to be trying to figure out how to swim efficiently in the air, passively adapting to what’s going on around them. Experiments show that the best “pilots” and “swimmers” on Earth, they can no longer use their best skills, but instead become the most frightened animal astronauts. Animals exposed to weightlessness are bound to suffer some effects, most notably the effect of gravity on movement. Those who have had pets may know that cats can walk normally even when blindfolded, but in microgravity, the test subjects can only hover or pan in a straight line, they didn’t stop until they hit something. You can see in the experiment the cat is like a side handstand while rotating the comical scene. Pigeons in microgravity also lose their ability to fly normally, flying erratically and as if in a panic because they can’t tell which way is up or down, as if in a mental confusion. Both cats and pigeons have very sensitive sense-positioning systems, constantly seek an objective up-and-down axis in microgravity, but it’s all for naught. Fish are an interesting example of how well they seem to adapt to swimming in microgravity. Their bodies are designed for swimming, and the buoyancy of water has much in common with weightlessness in the air. In fact, the fish seem to have coped well with the ordeal, even as they played with floating bubbles. Unlike cats, dogs are calm and stable in weightlessness, like drifting in a swimming pool, enjoying everything around them. It may have something to do with the temperament of the

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dog, in contrast to the cat that has a very nimble body, whereas the dog has a very good sense of smell but is not as good at jumping as the cat, so when faced with an extreme environment like weightlessness, instead, the dog goes with the flow and follow with the fun, so it floats steadily in space. At this point, a dog tells us a philosophy of life, in the face of unknown environmental shocks, no response is the best response. Monkeys are primates, with big brains and quick hands, and have long been considered close relatives of humans, so scientists used monkeys as the preferred experimental animals in space. The question of long-term human survival in space is at the forefront of current research: can the human body withstand the extreme overweight of a rocket? How is the long-term endurance limit of being in space? How much damage do cosmic rays in space do to life? To answer these questions, traumatic experiments are needed, so animal experiments can provide definitive answers to these questions. The monkey, as a close friend of man, undertakes exactly this mission, and becomes a pathfinder to the space. A long life in weightlessness will cause an abnormal blood circulation system, a large amount of bone calcium will be lost, muscle atrophy will also occur. To deeply explore these physiological realities, space monkeys, instead of humans, have undertaken the task of carrying out these studies. Biological experiments are also a regular part of space, and many countries now have projects for a space biology laboratory in the life sciences to study the effects of weightlessness and overweight on biological growth, development, metabolism, and genetics. Scientists also carried various animals, such as mice, geckos, snails, rabbit, crustaceans, as well as insects, into the space station to complete a variety of experiments. Animals have made a great contribution to our understanding of space life, and perhaps, in the future, they will also become indispensable partners in human space life (Fig. 10.2). Fig. 10.2 Laika was the first dog to orbit Earth. On Nov. 3, 1957, aboard the satellite Sputnik 2 of the Soviet Union. Credit TASS

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10.3 Risk of Cosmic Rays and Solar Wind Storm on Astronauts When we live and work on the ground, we usually don’t have to worry about the dangers from the sky, because there is an atmosphere on the surface of the Earth, the thickness of which extends up to about 100 km above sea level, not only does it provide us with oxygen to breathe, keeps the moisture in the air from evaporating, but it also has the often overlooked function of an atmospheric shield, which is to protect us from cosmic high-energy rays, solar storms, and the impact of other space debris moving in the Earth’s orbit. However, when astronauts perform routine maintenance and repair work outside the spacecraft, their bodies will be exposed to the outer atmosphere of the space environment, and their health will be exposed to a variety of hazards, including, for example, the high vacuum of outer space induced loss of pressure, hypoxia, weightlessness, extreme changes in temperature difference, which are deadly risk. Although there are exquisite high-tech EVA spacesuit protection, potential risks, and even unpredictable dangers, would be still present, with a very low probability of the most extreme event happened in a century. In other words, a spacesuit can block conventional radiation, and radiation leakage in extreme cases is not a zero-probability event. Scientists study how cosmic rays and the solar wind storm are produced, what they are made of and how they harm humans, which will help scientists find ways to make a prevention. What are the cosmic rays and the solar wind storm? Cosmic rays are charged, energetic particles from outer space, such as protons, nuclei, or electrons. About 89% are pure protons or hydrogen nuclei, 10% are helium or alpha particles, and 1% are heavy elements. Electron streams (like beta particles) make up most of the remaining 1%; gamma rays and ultra-high-energy neutrinos make up only a tiny fraction. The energy diversity of cosmic ray particles shows that cosmic rays have a wide range of sources. The source of these particles could be an explosion of massive stars in the distant universe, pulsar radiation, black holes or active galactic nuclei, or solar radiation, the physical mechanism of which is still being explored. Although the Sun seems to give Earth light and heat in a gentle way, its surface actually reaches a temperature of 5,500 °C. The distance from the Earth to the sun is about 150 million kilometers, and the effects of the sun can be felt in the heat of summer. The hot sun causes the protons and electrons of matter to separate into plasma motion. Driven by the magnetic field amplified by the Sun’s motion, a large amount of energy leaks into the Solar System, which carries a continuous supply of solar plasma, like the Earth’s magma spilling out, eventually forming what is known as a “solar storm”. Because of the protection of the Earth’s atmosphere, the effects of these solar storms are absorbed and life on Earth is protected from the harmful effects of solar radiation. However, the sudden solar storm can result in abnormal radio communications of civil aviation aircraft, and even affect the aircraft in adjusting the direction of flight, affect, around the Earth’s satellite operation and space station. The most recent solar storm, in 2012, wreaked havoc on electronics, costing the U.S. economy loss of trillions of dollars. Solar storms can cause satellites

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to fail in a number of ways, such as permanently degrading the output of solar panels rather than generating electricity, causing satellites to crash into the Earth’s atmosphere. The only solar storms visible to the naked eye are the colorful auroras, which are often viewed in the Earth’s polar areas. We can be sure that the environment on Earth is much better than that in space because of the double protection of the Earth’s magnetic field and dense atmosphere. All kinds of harmful rays and high-energy radiation are blocked out of the Earth’s atmosphere, most of the solar storms that hit the Earth are also neutralized by the Earth’s magnetosphere and atmosphere. Solar storms should not have a direct and serious impact on human health on the ground. Then, how terrible is the appearance of a solar storm in outer space environments. If intense sunlight, combined with solar storms, directly irradiates the spacesuit without blocking by the atmosphere, its ultraviolet light and high-energy particles are likely to damage the life-support system of the spacesuit, which may cause radiation the damage to the astronauts. At this time, if the total amount of radiation suffered by the astronauts is more than a certain human body limit, the harmful possibility has become prominent. Thus, high-energy radiation has become one of the major factors in the space station or long-term or long-distance spacecraft flights (Fig. 10.3).

Fig. 10.3 Solar storm observed by SOHO satellite in 2012. Credit ESA

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10.4 Plant Growth on Space Station The height of a tree depends on lots of factors, such as the soil moisture, abundant resource, nutrition, gene, water, and light, as well as the nearby environment conditions. The highest living tree existed on Earth is the redwood in California, with height of about 115 m, however what makes the height of the tallest tree on the Earth? The scientists estimate that a maximum tree height is of about 130 m, since the gravity and resistance of water transporting may ultimately control the leaf growth and photosynthesis that are responsible for the further height. As known, there are capillary tubes in the stem of plants, they can transport water from soil to leaves, by which the leaves on the tree top can also get enough water. But the ability to move water up has a limit, when this limit is reached, the plant cannot grow much taller. When the force of pulling water up is the same as the water weight, the balance is reached and the plant will stop grow. On the contrary, the capillary behaves differently on space station, since there is weightless there, so there’s nothing competing with the capillary action as what occurred on the ground. In principle, the plants can grow as long as possible if the space station is large enough, because the conception of tree height on space station is meaningless. As expected, in the space plant factory, plants use the transpiration to move water around their bodies effectively. The water and nutrition for plant growth are transported by tiny tubes in root, stem, and leaves, which is like a pump keeping water from ground to each part of a tree. Therefore, the space tree would be much different from those of their similar species on the earth.

10.5 Broad Knowledge Inside the Spacesuit What are our requirements for choosing clothes, warmth, or aesthetics? If we want to fly into space, these selection criteria are useless. Safety and reliability is the most important factor in spacesuit design, because it is not a normal suit, but a life-support protection system. In fact, to meet the security needs of outer space, a spacesuit could be worth as much as ten millions of dollars. This is amazing! It is because the outer space environment is too bad, which isn’t conducive to the life of astronauts, such as vacuum, hypoxia, high radiation, and big temperature difference, and these will seriously hurt the astronauts. So, how can we ensure the safety of their lives? In addition to the isolation from the outside world, safe and reliable spacesuits are particularly important. As we wear different clothes in different places or occasions, the astronauts’ clothes are divided into spacesuits, outer spacesuits, daily work clothes, and sports wears according to different functions. Spacesuits and sports wears are like astronauts’ leisure wears, mainly for the normal track. On the space, astronauts wore these kinds of suits and were easy for daily life and work.

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The cabin spacesuit must be worn when the spaceship is rising, returning, and joining. If the leakage occurred in the cabin or the pressure suddenly reduced, astronauts need to wear them in time, and connect to the cabin with the matching oxygen, then clothes will be immediately filled with gas, and provide certain temperature protection and communication functions to ensure the safe return. And extravagance spacesuits are even more amazing, people will be able to walk freely on space station with it. Isn’t it very strong? It can create a small suitable environment for astronauts, including the appropriate temperature and pressure, the adequate oxygen and ventilation, and control the human body balance to protect the astronaut’s vision. A piece of clothes can build a small world, and it is an independent life security system. We can see its precious in the following. When it comes to space suits, we can’t afford to talk about space jetpacks, and they are like cars’ power motor and are set on the astronauts’ backs. Astronauts can switch control on the nozzle of the backpack on the arm of your chair, ejecting compressed nitrogen in the backpack, thus forming different sizes of reverse thrust in all directions, implement different direction of movement. With it, astronauts can be like Astro Boy and the Monkey King, and turn somersaults, spin, up and down, forward and backward in the vast space. Some children worry about astronauts: when they wear heavy clothes, although they can walk freely, can they work like Astro Boy? It is not necessary to worry that the researchers who made spacesuits have taken this into account. The spacesuits are usually made of clothing, helmets, gloves and boots. Clothing in the joints have done a very humane treatment, and will not affect the bending, flip, and other acts. Especially for the astronauts’ gloves, they are connected through the wrist and clothing. Gloves are manufactured by the astronaut’s individual hand, and the finger joints are corrugated structure, such as the accordion bellows, so astronauts can bend freely and operate the equipment flexibly. Is the astronaut’s equipment full of knowledge and like a treasure house? The spacesuit used for space walking is a personal airtight equipment to guarantee the astronauts’ life activities and working ability, which provides all the protection that the atmosphere does for the inhabitants of the Earth. The spacesuit satisfies five basic functions, such as provides oxygen, CO2 removal, radiation protection, temperature control, and pressure control, which allows astronauts to survive safely in space for up to 9 h. In space, the temperature difference between sunlight and shadow can be over 200°, so the outer shell of the boots is made of many layers of radiation-resistant, high-and low-temperature resistant materials. In a vacuum, the nitrogen contained in the body’s blood becomes a gas, causing the volume to expand. If people do not wear pressurized airtight spacesuits, the pressure difference between the body and the outside results in life-threatening. Meeting so many safety requirements, spacesuits have become unwieldy and immobile, weighing as high as 90–130 kg and virtually impossible to walk on the ground. Fortunately, in weightlessness in space, astronauts don’t have to handle it very hard (Fig. 10.4).

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Fig. 10.4 In space walking, the expedition 54 commander Alexander Misurkin and flight engineer Anton Shkaplerov spent eight hours and 13 min outside the International Space Station. Credit TASS

Test of space suit in weightless sink. Astronauts extravehicular activities are important technical components. It can complete the spacecraft on-line maintenance and troubleshooting, payload placement, recycling and space station in orbit installation and construction. The completion of these missions requires the astronaut to wear the outer space suit in the weightless environment. The motion and operation of the weightless environment are completely different from those on the Earth’s surface. However, due to the Earth’s gravity, it is impossible to feel the true weightless state on the ground for a long time. Therefore, if you want to get the astronauts to successfully complete the extravehicular activities, you will need to carry out a professional weightless training method, the weightless sink. The working principle of the weightless sink is buoyancy trim. The astronauts wearing spacesuit are immersed in the water. By placing them in the water, the buoyancy is equal to the gravity, which makes it neutral buoyancy. If they want to go up, they can take a sip of oxygen, and they breathe out, then they can go down, which is very similar to the weightlessness environment. But the water resistance in the sink will affect the astronauts’ movement, so their training in the sink often looks like “playing tai chi”. The water in the sink is very clean and can even be consumed directly (Fig. 10.5). Fig. 10.5 Weightlessness training in the water sink for space walking. Credit China CCTV

Chapter 11

Postscript: Flying into Space is a Human Dream

As the book was being written and handed in, I took a long breath of fresh air, like an astronaut returning safely to Earth from space, and suddenly felt a sense of relief and a return to nature. Looking around, however, I notice that the COVID-19 coronavirus epidemic is ongoing, together with the war between Russia and Ukraine without stopping, and that these traumatic events have accompanied my writing, meanwhile my mother also died during this period, which undoubtedly slowed down my manuscript preparation process, but also increased my digest thinking. As a scientific researcher and teacher, let the public all over the world, especially children, learn and understand the space environment knowledge, share the fun of exploring the outer space, appreciate the wonderful phenomena of the universe, and awaken the conscience of people to cherish our common home—the Earth. Well, it is my strongest desire to transcend the space dream at this moment, and wish the space station to become a spiritual refuge for human beings to seek permanent peace. Fortunately, thanks to the joint efforts of scientists, the International Space Station is still continuing to operate; In addition, the refined construction of the China space station has also begun and is embarking on a new journey, in which the Chinese astronauts have also carried out the science popularization activities in the “Tiangong Class” that has brought the live broadcasts to the world (Fig. 11.1). The significance of the space class is obvious, and it inspires the public and children yearning for the science and the sky, extending their sight and increasing their knowledge (Fig. 11.2). It is unbelievable that the National Aeronautics and Space Administration (NASA) has prepared the Artemis plan to return to the moon in a few years, which will start a new round of lunar missions by sending the first woman to make the next giant leap on the lunar surface. Similar to NASA, China and Russia also announced plans to build a lunar base and explore the moon. Humans will establish living and working areas on the moon to better carry out scientific experiments, astronomical observation, and other living activities. Scientists pointed out that it is necessary to conduct an allround exploration of the moon and gradually develop its resources. It is now known that the moon contains extremely rich mineral resources and helium reserves. There © Springer Nature Singapore Pte Ltd. 2023 C. Zhang, An Adventure, https://doi.org/10.1007/978-981-19-9221-6_11

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90 Fig. 11.1 In China’s space station, astronauts performed the popular science live lecture, named as “Tiangong Class”, which attracted billions of audiences worldwide

Fig. 11.2 As a live interpretation guest for the first “Tiangong Class”, the author (Dr. Chengmin Zhang) showed on the screen of CCTV broadcast. Credit CCTV/China

11 Postscript: Flying into Space is a Human Dream

11 Postscript: Flying into Space is a Human Dream

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Fig. 11.3 Human beings as the products of the Earth are confronting the global environmental crisis and thinking about where to go in the future

is no doubt that it will provide timely help for the Earth with energy shortage in the future. With the globalization of the twenty-first century, human activities expand from land, sea, and sky to outer space. We are also faced with environmental degradation, energy crisis, extreme climate, global warming, and population explosion. It will be an urgent public topic to search for another Earth and extraterrestrial civilization in space. Finally, when we finish reading this book, let us remember that the human flight dream will never lose weight, and the tension of the scientific dream is unlimited. I hope this book can light up your scientific dream (Fig. 11.3).