Re: How far exactly does the Earth's atmosphere extend into space?

Hi Julie!

Well, the Earth's atmosphere doesn't have a clearly defined boundary, as you probably know. Furthermore, it depends somewhat on what you call an "atmosphere." But we can take a few examples.

Let's say we define "atmosphere" to mean relatively dense gas, with a variety of molecular species present. This region extends to a mere 500 km above the Earth's surface. (That's about 55 times the height of Mt. Everest, to give you a sense of scale.) The moon is considerably further than that !

So let's say we include in our definition of "atmosphere" any region containing particles that are held to the Earth by its gravity. This region extend to perhaps 1000 km -- still extremely close compared to the distance to the moon.

Finally, let's say we consider any kind of particle associated clearly with the planet Earth -- this now includes charged particles ("ions") that are held to the Earth not only by gravity but by interactions with our magnetic field. This includes particles like free electrons in addition to ions. This region is called the "ionosphere" and exists between roughly 70 -- 2000 km. But even this outermost layer of what we might mean by "the Earth's atmosphere" is well inside the orbit of the moon: the moon orbits at a distance of roughly 384,000 km!

Tess Lavezzi