Earth's atmosphere stretches further than we realized - to the Moon and beyond.
It turns out that Earth's gaseous layers reach up to 630,000 kilometres away, or 50 times the diameter of our planet. To put this in perspective, that places the moon well within Earth's atmosphere.
Researchers only just discovered the find after pouring over data collected by the ESA/NASA Solar and Heliospheric Observatory, or SOHO, which orbits some 1.5 million kilometres from Earth towards the Sun. The satellite is equipped with an instrument known as SWAN, which has a hydrogen absorption cell capable of detecting the sparse outer layer of Earth's atmosphere, which is really just a cloud of hydrogen in its furthest reaches.
The cloud of hydrogen that makes up the far-out atmosphere is known as the geocorona, and it glows under a certain wavelength of ultraviolet light when the Sun is shining on it, almost like an ultraviolet rainbow. It's this gleam that SWAN was uniquely capable of detecting, to trace the true outline of Earth's geocorona.
The outer geocorona is thin, with only about 0.2 atoms per cubic centimetre at the moon's distance, so it wouldn't be noticeable to most spacecraft flying through it. Even so, the find might put some limitations on our orbiting telescopes, or any future telescopes that might be placed on the moon. Space telescopes observing the sky in ultraviolet wavelengths to study the chemical composition of stars and galaxies now need to take this into account. We may not be getting as clear a view as we thought when we gaze outside our own Heliosphere.
This find could give us new ways of detecting potential reservoirs of water beyond our solar system, because our hydrogen exosphere is most likely a result of having so much water vapor closer to our planet's surface. From now on we might recognize other Earth-like planets based upon their gleaming geocoronas.
An interesting way of thinking about this finding: this essentially means that no humans have ever left Earth's atmosphere, even counting astronauts who walked on the lunar surface.
Source: Mother Nature Network: https://www.mnn.com/earth-matters/space/stories/earths-atmosphere-stretches-further-we-thought-possible-moon-and-beyond