Scientific research has found that the moon, 380,000 kilometers away, is actually still in the earth's atmosphere.

How thick is the earth's atmosphere? This question seems simple, but even scientists can't answer it accurately. The definition of the earth's atmosphere has always been controversial. In the study of the earth's atmosphere, scientists even found that the moon is actually in the earth's atmosphere. This discovery can be said to have completely subverted people's cognition. How can the distant moon, which has always been recognized as outer space, still be in the earth's atmosphere?

In the aerospace field, it is very important to know the thickness of the atmosphere, because for spacecraft, only by breaking through the earth's atmosphere and getting rid of the air resistance of the earth can they really enter space outside the earth. This kind of flight activity is also called spaceflight.

The flight (navigation) activities of aircraft in the earth's atmosphere (airspace) are called aviation.

According to the definition of the International Aviation Federation, the boundary line from "aviation" to "aerospace" is called "Carmen Line", and this height is 100KM. If any spacecraft wants to fly continuously, it must break through the Carmen line. When the tangential speed of the Carmen line reaches above the first cosmic speed, it can fly continuously around the earth.

Although the height of the Carmen line has been defined by the International Aviation Federation, there is still great controversy among scientists about the determination of the height of the Carmen line. From 195 1 to 1962, there are more than 30 different opinions about the height of Carmen line. These boundary heights range from 20 km to 400 km above sea level, but most of them are between 75- 100 km.

In fact, it can be seen from various human space activities that the thickness of the earth's atmosphere can far exceed 100KM. For example, the flying height of the International Space Station is about 400 kilometers, but during the flight around the earth, the height of the space station will continue to decrease. This is because there is still friction between the thin atmosphere and the space station at an altitude of 400 kilometers, which leads to the gradual decrease of the speed and orbital height of the space station. Therefore, there will be propellers in the space station to accelerate the space station regularly to maintain the orbital height.

How thick is the earth's atmosphere? Scientists try to define the earth's atmosphere with more universal standards, but the earth's atmosphere is actually much more complicated than imagined, far from being defined by a single line.

Astronomer Lyman Spitzer defined the concept of "escape layer", also known as "outer layer" and "escape layer", that is, the atmosphere above the thermosphere (warm layer) is the outermost layer of the earth's atmosphere. Under the action of solar ultraviolet rays and cosmic rays, most molecules in this layer of air are ionized; The content of protons and helium nuclei greatly exceeds that of neutral hydrogen atoms. The air in the escape layer is extremely thin and its density is almost the same as that in space, so it is often called the outer atmosphere. This atmospheric region is also called the corona of the earth.

Because the corona is located in the outermost layer of the atmosphere, if we can know where the outer boundary of the corona is, then we can know the thickness of the earth's atmosphere. What scientists do is to detect the area of the corona by observing the light emitted by the corona in outer space by spacecraft.

The principle of this method is that the hydrogen in the earth's corona will scatter with the far ultraviolet radiation of the sun and emit light. There are several kinds of spectral lines emitted by the earth's corona, among which the strongest spectral line is Lyman α radiation, which is the spectral line emitted when the electrons of hydrogen atom jump from principal quantum number n = 2 to n = 1. Researchers mainly use it to detect the corona. The equipment used by scientists is the Solar and Heliospheric Observatory (SOHO). The equipment was jointly developed by the European Space Agency (ESA) and the National Aeronautics and Space Administration (NASA) of the United States. Instruments on SOHO can filter out Lyman alpha radiation from outer space and accurately measure light from the corona of the earth.

It is found that the circumference of the earth's corona far exceeds people's expectations, and the height of the outermost layer reaches nearly 630,000 kilometers, while the average distance between the earth and the moon is 380,000 kilometers, which is equivalent to the fact that the moon is still in the earth's corona, in other words, the moon is still in the earth's atmosphere.

In addition, the coronal region will also be affected by sunlight. Under the action of solar light pressure, the corona is distributed like a comet tail, and the light pressure on the side facing the sun is relatively high, which makes the hydrogen atoms in the corona layer "compressed" by sunlight. Through the analysis of the detection results, it is found that there are about 70 atoms per cubic centimeter at a distance of 60 thousand kilometers from the surface, while at a distance of 380 thousand kilometers (about the orbital height of the moon), there are only 0.2 atoms per cubic centimeter on average. On the side facing away from the sun, the density of hydrogen atoms is larger as a whole, and the corona range will be larger.

By observing the corona of the earth, scientists have obtained more accurate data of the earth's atmosphere. Although the content of the earth's atmosphere is negligible at the height of the moon, scientists have a more accurate understanding of the earth's atmosphere.

reference data

[1]SWAN/SOHO Lyman‐α drawing: the hydrogen earth corona is far beyond the moon [J]. Journal of Geophysical Research: Space Physics, 20 19, 124(2):86 1-885.