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Contrary to general belief that Earth's atmosphere stops a bit over 62 miles from the surface, a new study based on observations made over two decades ago by the joint US-European Solar and Heliospheric Observatory (SOHO) satellite shows that it actually extends as far 391,000 miles (630,000 km) or 50 times the Earth's diameter. This makes the Moon a very high altitude aircraft. From a report: Launched on December 2, 1995 atop an Atlas IIAS launcher from Cape Canaveral Air Force Station in Florida, SOHO is parked in the first Lagrange point (L1) 930,000 miles (1.5 million km) from Earth where it has carried out studies of the Sun and the solar winds, and will continue to do so until at least 2020. From this vantage point, the observatory's Solar Wind Anisotropie (SWAN) instrument is able to measure the presence of hydrogen by looking at the Lyman-alpha line in the solar spectrum. And what works for the Sun, works for Earth. By turning SWAN on the Earth at the right times of the year, SOHO was able to detect hydrogen atoms from the atmosphere and measure how far out they extend into what space scientists call the geocorona. While the existence of the geocorona is well known -- the telescope set up by the Apollo 16 astronauts on the Moon even photographed it -- no one was sure how far out it reaches until now.
By looking at data collected by SOHO in the mid 1990s, scientists from Russia's Space Research Institute and elsewhere were able to work out the extent and density of the geocorona. What they found was that sunlight on the day side of the Earth compresses the hydrogen until it reaches a density of 70 atoms per cubic cm at an altitude of 37,000 miles (60,000 km), and on the night side it can expand out until it has a density of only 0.2 atoms per cubic cm at the distance of the Moon's orbit. According to the study leader Igor Baliukin, the geocorona is so tenuous that it poses no hazard to astronauts or spacecraft.
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