The Earth's atmosphere consists of several layers: the troposphere (upper limit 20 km), stratosphere (border 50 km), mesosphere (border 85 km), thermosphere (border 690 km) and exosphere (border 10,000 km). For a long time, the so-called Karman line, located at an altitude of 100 kilometers, has been taken as a conditional boundary between the Earth's atmosphere and space. However, in the course of a new study, the results of which were published in the Journal of Geophysical Research: Space Physics, it was found that the atmosphere of our planet is much more complex than it might seem at first glance. Scientists have found that its borders go far beyond the limits of the moon.
The space, including the Moon and which is the outer part of the uppermost layer of the Earth's atmosphere, the exosphere, is called the geocorona by researchers. It is a cloud of hydrogen atoms that begins to glow when exposed to ultraviolet radiation. Since this cloud is very thin, measuring its actual boundaries proved to be a challenge. Thus, according to the results of previous studies, the upper boundary of this space was determined by the distance of about 200,000 kilometers from the Earth, the point beyond which the pressure of the solar wind already overrides the force of the Earth's gravity.
An international scientific group led by Igor Balyukin from the Space Research Institute of the Russian Academy of Sciences, using data collected by the SOHO (Solar and Heliospheric Observatory) spacecraft, which is a joint project of the European Space Agency and the American aerospace agency NASA, was able to find out that the previously established geocorona boundary does not even come close to the real one. state of affairs. Researchers have established that the geocorona is actually at least 630,000 kilometers long. In other words, this means that the boundaries of our atmosphere are far beyond the boundaries of the Moon, which in turn is only 384,000 kilometers from our planet.
The boundary of the Earth's geocorona is marked in blue (not to scale).
This discovery is made even more interesting by the fact that it was made on the basis of observational data conducted from 1996 to 1998, that is, more than 20 years ago. All this time they lay in the archive, awaiting analysis.
The data was obtained using the spacecraft's highly sensitive SWAN instrument, designed to measure the far ultraviolet radiation of hydrogen atoms, called Lyman-alpha photons. It is impossible to see them from the Earth - they are absorbed by the inner layers of the atmosphere, therefore observations must be carried out directly in space. For example, the Apollo 16 astronauts were able to photograph the geocorona in 1972.
A photograph of the Earth's geocorona taken from the Moon by the Apollo 16 astronauts.
The SWAN instrument has the advantage of being able to selectively measure geocorona radiation by filtering out Lyman-alpha radiation from deep space. This is what allowed scientists to create a more accurate map of this part of the earth's atmosphere.
The new study not only helped to understand the true size of the geocorona, but also showed that the pressure from sunlight increases the density of hydrogen atoms on the daytime side of the Earth and creates an area of increased density on the night side. Nevertheless, even on the daytime side, this density is quite low - at an altitude of about 60,000 kilometers above the surface of the planet, it is about 70 hydrogen atoms per cubic centimeter. On the night side, it is even lower and continues to decrease down to 0.2 atoms per cubic centimeter as it approaches the circumlunar orbit.
The good news, the study authors explain, is that these particles will not pose any additional threat to astronauts on future manned missions to the Moon.
The bad news is that the geocorona may interfere with future astronomical observations that will be made near the Moon.
The last one can be noted one interesting fact. If the research data are correct, then from a technical point of view, even in the conditions of space launches, a person has never left the Earth's atmosphere.