Strictly speaking, there are no strict boundaries between the Earth and space. Our atmosphere does not end at a certain altitude; it goes out gradually. A new study by the Russian Space Research Institute (SRI) shows that our atmosphere spans 630,000 km in space.
The main author of this study is Igor Baliukin. researcher at Russia's SRI, Department of Small Bodies, Systems and Physics, Solar System. Jean-Loup Bertaux, of LATMOS at the University of Versailles-Saint-Quentin-en-Yvelines, France, also participated in the study. The study used archival data from the Solar and Heliospheric Observatory (SOHO) to find the gaseous extension of the Earth's atmosphere.
This study is about what is called geocorone. It is a vast cloud of hydrogen atoms located at the junction of Earth's atmosphere and space. SOHO has 12 scientific instruments on board, one of which is called SWAN (Solar Wind Anisotropies). SWAN was able to track the geocorone's hydrogen signal and detect its outer limits with unparalleled accuracy.
The Apollo 16 astronauts actually took pictures of the geocorone with the first camera on the lunar surface in 1972. But at the time, they did not know they were still in the Earth's atmosphere.
"The Moon flies in the Earth's atmosphere. "
Igor Baliukin, Russian Center for Space Research.
This study also focuses on what is known as Lyman-alpha light. It is a particular ultraviolet wavelength that interacts with hydrogen atoms. Atoms can both absorb and emit this light. The problem is that in the earth's atmosphere this light is absorbed. The only way to see the extent of the crown is in space. Even in this case, SWAN / SOHO observations can only be made at certain times of the year, when the Earth and its geocorone wheel are in view of the observatory.
SWAN's design allows it to measure hydrogen atoms in geocorona and to eliminate or reject hydrogen atoms in space.
Scientists behind the new study found that sunlight squeezes hydrogen atoms on Earth's day and also produces increased overnight density. However, this density is only relative; the dense region of the day has only 70 atoms per cubic centimeter at 60,000 km above the Earth. At the distance of the Moon, there is only 0.2 atoms per cm3.
"The Moon flies in the Earth's atmosphere, "Said Baliukin, lead author of the paper presenting the results. "We did not know this before dusting off observations made more than two decades ago by the SOHO satellite.. "
Although the geocorone extends far enough to encompass the Moon, this does not mean that it would facilitate space exploration. Although hydrogen is an extension of the atmosphere, the density of hydrogen atoms is still so low that it is rather a vacuum. But that does not eliminate this result.
"On Earth, we would call this emptiness. This additional source of hydrogen is therefore not important enough to facilitate space exploration., Igor said.
But it is important when it comes to exoplanets. For planets containing hydrogen in their exospheres, the water vapor often appears closer to their surface. This is the case for the Earth, Mars and Venus. This fact might be helpful in trying to determine which exoplanets might have water.
"This is particularly interesting when looking for planets with potential water reservoirs beyond our solar system."Explains Jean-Loup Bertaux, co-author and former principal investigator of SWAN.
This prolonged atmosphere and its ultraviolet radiation pose no danger to astronauts on mission in this region of space. There is also ultraviolet radiation associated with geocorone, since hydrogen atoms scatter sunlight in all directions, but the impact on astronauts in lunar orbit would be negligible compared to the main source of radiation – the Sun., Says Jean-Loup Bertaux.
But it is possible that the geocorone interferes with the astronomical observations made near the Moon. This is something that any lunar telescope should take into account. "Spatial telescopes observing the sky in ultraviolet wavelengths to study the chemical composition of stars and galaxies should take this into account."Adds Jean-Loup.
SOHO was launched in 1995 and has been studying the sun for over 20 years. He is still in orbit around the L1, even though it was designed for a two-year mission. During his life, he has to his credit several "firsts".
SOHO's SWAN instrument observed the Earth's geocorone three times between 1996 and 1998. The team decided to retrieve these data from the SOHO archives and analyze them further. This discovery makes us wonder what other discoveries are hidden in his archives.
"Data archived many years ago can often be used for new science"Says Bernhard Fleck, scientific leader of ESA's SOHO project. "This discovery highlights the value of data collected more than 20 years ago and SOHO's outstanding performance. "
The new study is published in the Journal of Geophysical Research: Physics of Space.