空间科学概论(英文版)

Chapter 1 Reasons to Conduct Research in Space 1.1 Introduction What are the reasons to conduct research in space? For many disciplines， even including astronomy， research could be carried out on the ground. For example， Galileo Galilei (1564-1642) pioneered the practical ground observation of celestial bodies using telescopes. Another example is the employment of ground-based radars to observe and study the ionosphere. Even so， there’s still a lot of research that can’t be done on the ground， which necessitates the research in space. This chapter will focus on the reasons to go into space. From the beginning of the space age， the fundamental and foremost objective of carrying out research in space is to unveil the mystery of space and increase our knowledge of space. Before the launch of the first artificial satellite in 1957， the outer space reaching beyond the atmosphere is shrouded in mystery， where the neutral atmosphere thins out and is ionized by the ultraviolet light from the Sun when it reaches further out， hence creating the ionosphere. But， questions remain to be answered， e.g.， how the electrons and ions in the ionosphere are distributed and how do they move? What effect does the Earth’s magnetic field exert on these charged particles? After gaining access to space， for the first time in human history， we have the opportunity to observe the Earth from hundreds or even thousands of kilometers away. When we observe it from such a distance， our perceptions of the Earth become very different. The changes that the Earth presents to us become systematic， such as the formation and movement of typhoons. What’s more， once breaking free from the obstacles of the atmosphere， we have the liberty to make full use of the resources of the entire electromagnetic spectrum. The low-frequency electromagnetic waves， terahertz， and infrared wavelengths， as well as wavelengths beyond the ultraviolet are normally blocked by the atmosphere. Entering space enables us to observe the universe in the full electromagnetic spectrum. For an in-orbit spacecraft， the centrifugal force generated by its rotation around the Earth is offset by the gravitational force of the Earth， providing us with an equivalent microgravity environment for a long period of time. Previously， our understanding of the kinetic properties of matter and the rule of life activity is actually based on the condition of the gravity of the Earth. So， if we remove the gravity， will the movement of matter and life remain the same? In short， gaining access to space is to enter a larger laboratory where the experiments previously impossible on the ground can be carried out. 1.2 To Explore the Unknown Space Environment Before the space age， the human knowledge of space was limited to speculations and theoretical conjectures. The atmosphere thinned out， but then what? The ultraviolet light from the Sun ionizes atoms in the atmosphere， allowing electrons to escape and correspondingly form the ionosphere. The answer was not clear in 1901， when Guglielmo Marconi (1874-1937)， an Italian radio engineer， successfully transmitted a radio signal across the Atlantic. Marconi was puzzled for a long time by the fluctuation of radio waves， which apparently traveled a winding path to reach the destination thousands of kilometers away. We now know that， for a transmission distance of more than 5，000km from the west coast of Europe to the east coast of the United States， the radio waves reached the receivers with the help of ionospheric reflections. It turns out that Marconi’s first successful transoceanic radio communication in 1901 verified the existence of the ionosphere. The human knowledge about the ionosphere stopped there. By that time， we still did not know where the upper boundary of the ionosphere was， or how positively charged ions and negatively charged electrons in the ionosphere behaved. Only after 1957 did the answers to these questions became clear. Therefore， to study the unknown space environment is the core of space research. This is especially the case for the first artificial satellite launched by the Soviet Union on October 4， 1957， and the first American artificial satellite launched on January 31， 1958. Malfunctions were detected on the instruments for both satellites and American scientists tended to believe that the malfunctions were not due to the instrument itself but rather to the existence of an intense high-energy particle zone in the near-Earth space， which was later identified and consequently named as Van Allen belt. This is the first major discovery in the space history of mankind. 1.3 To Break Free from the Barrier of Atmosphere to Electromagnetic Wave Since Galileo pointed his telescope into space， human beings have broken the limita-tions of space observation with the naked eye and began to use scientific instruments to observe the universe. The spectrum of electro