Why are space stations and satellites of various countries all concentrated in orbits at an altitude of 400 kilometers?

Not long ago, I was asked a very interesting question: Why did our Tiangong Space Station, like the International Space Station, choose an orbit close to 400 kilometers in altitude? Is there anything special about this altitude?

This observation is particularly detailed. In fact, there are many satellites operating at an altitude of several hundred kilometers. This is also the space region with the densest distribution of artificial satellites. For example, the famous Hubble Space Telescope is also at an altitude of about 500 kilometers. Therefore, people must always monitor various spacecraft in this area, including space debris, to prevent them from colliding.

So why don't we design satellite orbits to be more dispersed to reduce the probability of collision? For example, what would happen if we lowered the satellite orbit? It's easy to imagine, because the density of the atmosphere decreases exponentially with altitude. So if the satellite orbit drops below 300 kilometers, the atmospheric drag will increase significantly, and the satellite's speed will drop rapidly, causing the satellite to continue to decrease in altitude until it crashes. So, can we design the satellite orbit to be higher?

This is actually possible. There are indeed some satellites operating in higher orbits. However, at this time, we will face another threat, which is radiation in space. Speaking of it, radiation is not a mysterious thing. We are exposed to various types of radiation in our daily lives. Sunlight, wireless communication, these are actually radiation. But in this kind of radiation, the energy of each photon is very small and does not harm us. But higher energy particles, such as X-ray photons, or high-energy charged particles in space, have enough energy to ionize atoms, that is, to knock out electrons in atoms. If people are exposed to this kind of ionizing radiation environment, it will cause damage to the central nervous system and the hematopoietic function of the bone marrow, and will also increase the risk of cancer. High-energy particles can also cause the charging and discharging of satellite components, directly damaging the instruments, and sometimes cause some components to jump in potential state, such as from 0 to 1, or from 1 to 0, resulting in incorrect calculation results, and if you are unlucky, it will also cause the system to crash. So these particles are sometimes called "killer" particles of satellites.

The picture comes from Tuchong.com

There are a large number of high-energy particles in the space near the earth, and this area is called the radiation belt. In fact, the radiation belt is the first scientific discovery after humans entered the space age. As we all know, in the 1950s, the Soviet Union and the United States launched artificial satellites one after another and started the space race. On some of the earliest satellites, both the Soviet Union and the United States installed a particle detector specifically for measuring ionizing radiation, called a Geiger counter. Soon, American scientist Van Allen discovered that the radiation dose in space was far beyond previous imagination, and even exceeded the range of the Geiger counter. So people later named the radiation belt after Van Allen, and called it the Van Allen radiation belt.

Now we know that the high-energy particles in the Van Allen radiation belts are actually trapped in the space near the Earth by the Earth's magnetic field and bounce back and forth between the two magnetic poles. Of course, these high-energy particles are not easy to reach the Earth's surface. On the one hand, the closer to the Earth, the stronger the magnetic field, and these particles are easily bounced back. On the other hand, because of the existence of the atmosphere, these particles will collide with atmospheric molecules or atoms and lose energy. So in general, as the altitude increases and the atmospheric density decreases, these high-energy particles in space will increase rapidly, posing a huge threat to satellites and astronauts.

Therefore, everyone will try not to let the satellite fly too high to avoid the influence of the radiation belt. Of course, it cannot be too low, otherwise it is easy to crash. Then this altitude of several hundred kilometers becomes the most ideal orbit. But sometimes people have to let the satellite fly farther, such as to the moon, or need to be fixed above a certain point on the earth, that is, to reach the geosynchronous orbit at an altitude of more than 30,000 kilometers, then the radiation problem must be considered more seriously. Of course, we can increase the thickness of the protective armor, but after all, the weight of the spacecraft is limited, and the armor cannot be increased indefinitely. So people need to have a deeper understanding of the radiation belt, for example, what is the source of these particles? How are they accelerated? What is their distribution in space?

Also, if there is solar activity, how will the distribution of these high-energy particles change? Now the major space powers in the world are conducting research in this area, and my country has also launched a space weather forecast service, which includes the distribution of high-energy particles in the radiation belt. In this way, at critical moments, we can take some specific evasive actions, or at least shut down some instruments to avoid greater losses.

After all, we can't let these tiny particles stop us from running towards the sea of ​​stars.

This article is a work supported by Science Popularization China Starry Sky Project

Author: Zhou Xuzhi

Reviewer: Liu Yong (Researcher at the National Space Science Center, Chinese Academy of Sciences)

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.