The wispy outermost layer of Earth’s atmosphere extends much deeper into space than scientists realized — deep enough that the moon orbits through it.
Earth’s geocorona is a sparse, little-understood collection of hydrogen atoms loosely bound by gravity to our planet. This atmospheric region is so thin that on Earth we’d call it a vacuum. But it’s important enough, and powerful enough, to mess with ultraviolet telescopes due to its habit of scattering solar radiation. And researchers, looking at old data from the 1990s, now know that it extends up to 400,000 miles (630,000 kilometers) above the planet’s surface. That’s between 10 and 25 percent farther than previous estimates.
One of the reasons the geocorona is so little understood is that it’s hard to find a vantage point from which to study it. From Earth’s surface and even low Earth orbit, it’s more or less invisible. The most famous image of it (pictured above) comes from the 1972 Apollo 16 mission, when the moon, Earth and sun aligned in such a way that astronauts were able to snap a photo of sunlight scattering through it.