Scientists discover Earth’s atmosphere goes all the way out to the moon

21 Feb 2019

Image: © sdecoret/

After coming across 20-year-old data, astronomers have found that Earth’s atmosphere is much larger than we thought, even as far out as the moon.

When Neil Armstrong and Buzz Aldrin touched down on the lunar surface in 1969, little did they know that they were actually embedded within Earth’s atmosphere. That’s according to new findings reported by scientists at Russia’s Space Research Institute.

The discovery – found in data collected by the orbiting Solar and Heliospheric Observatory (SOHO) more than two decades ago – revealed that the gaseous layer of atmosphere that surrounds our planet actually reaches up to 630,000km away. This means it is not only 50 times the diameter of Earth, but it includes the orbit of our planet’s moon.

At the point where our atmosphere merges with outer space, there is a cloud of hydrogen atoms called the geocorona. The sun interacts with these atoms through a particular wavelength called Lyman-alpha, which the atoms both absorb and emit. Because this type of light can only be absorbed by the atmosphere, it can only be seen from space.

That is why SOHO has an instrument able to trace this hydrogen signature and precisely detect how far the very outskirts of the geocorona are. This was able to show that sunlight compresses hydrogen atoms on Earth’s dayside, while producing a region of enhanced density on the nightside.

Infographic showing the extent of Earth's atmosphere, SOHO and the moon.

An infographic (not to scale) showing the extent of Earth’s atmosphere. Image: ESA

Astronauts not under threat

In the dayside, hydrogen is quite spare, with just 70 atoms per cubic centimetre at 60,000km above Earth’s surface, and about 0.2 atoms at the moon’s distance. Thankfully for current and future astronauts, these particles don’t pose any kind of threat.

“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,” said Jean-Loup Bertaux, co-author of the paper published to the Journal of Geophysical Research.

However, Earth’s large geocorona could significantly impact future astronomical observations undertaken in the vicinity of the moon, meaning space telescopes would need to take this into account when reading measurements.

First launched in December 1995, SOHO orbits 1.5m km from Earth where it has been studying the sun, from its deep core to the outer corona and the solar wind. Speaking of this latest discovery, SOHO project scientist Bernhard Fleck of the European Space Agency said it was a somewhat fortuitous find.

“Data archived many years ago can often be exploited for new science,” he said. “This discovery highlights the value of data collected over 20 years ago and the exceptional performance of SOHO.”

Colm Gorey was a senior journalist with Silicon Republic