Observations show the distant cloud of hydrogen atoms that make up the outermost part of Earth’s atmosphere stretches beyond the Moon.

So, according to a new study, this gaseous layer that wraps around Earth, known as geocorona, reaches up to 391,464 miles (630,000 kilometers) away, or 50 times the diameter of our planet. Scientists based the study on observations made more than 20 years ago by the ESA/NASA Solar and Heliospheric Observatory, SOHO.

“The moon flies through Earth’s atmosphere,” says Igor Baliukin of Russia’s Space Research Institute, lead author of a paper presenting the discovery.

“We were not aware of it until we dusted off observations made over two decades ago by the SOHO spacecraft.”

The new study could help guide our search for watery planets outside of our solar system.

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One of the SOHO’s instruments, SWAN, used its sensitive sensors to trace the hydrogen signature and precisely detect how far the very outskirts of the geocorona are.

The presence of hydrogen in a planet’s exosphere is often a sign that water vapor exists closer to their surface.

The first telescope on the Moon, placed by Apollo 16 crew in 1972, captured an image of the geocorona surrounding Earth. You can see it glowing brightly in ultraviolet light.

Copyright: NASA

At that time, the astronauts on the lunar surface did not know that they were actually embedded in the outskirts of the geocorona,” explains Jean-Loup Bertaux, co-author and former principal investigator of SWAN.

The new study shows how sunlight causes geocorona’s hydrogen atoms to differ in density on Earth’s dayside and nightside.

SWAN instrument on the SOHO spacecraft made the observations which traced hydrogen signatures throughout the geocorona. Credit: ESA/NASA/SOHO/SWAN

So, the denser dayside region of hydrogen is still rather sparse, with just 70 atoms per cubic centimeter at 60,000 kilometers above Earth’s surface and about 0.2 atoms at the Moon’s distance.

On Earth, we would call it vacuum, so this extra source of hydrogen is not significant enough to facilitate space exploration,” says Igor.

However, these particles do not pose any threat for future space travelers orbiting the Moon.

There is also ultraviolet radiation associated to the geocorona, as the 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 according to Igor, ‘Space telescopes observing the sky in ultraviolet wavelengths to study the chemical composition of stars and galaxies would need to take this into account.’

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Thumbnail image: Earth’s geocorona. Credit: ESA