How tall is Earths atmosphere?

[jkool702]

So I realize that Earths atmosphere doesnt just "cut off" like in KSP, rather there is a gradual transition into less and less air pressure until it is effectively the same vacuum as in interplanetary space. In KSP i tend to think of the edge of the atmosphere at the point in which orbits are indefinitely stable, so I guess what I'm actually asking is:

"At what height would a satellite in an circular equatorial orbit of Earth need to be such that the orbit does not effectively decay over the course of many, many years (say a human lifetime)?"

Some numbers I found from a quick google search

100 km: where "space starts"

380 km: googles answer to "how tall is earth's atmosphere"

400 km: ISS orbit height

~600km: end of thermosphere / start of exosphere

10,000 km: end of Earth's Exosphere, where it is pretty much impossibly to distinguish the air from the vacuum of interplanetary space. (seems to be some variations on this altitude?)

Since the ISS isnt entirely stable (needs help to stay in orbit), I'd guess the answer to my question is somewhere between 400km and 10,000km. I guess 10,000km would be the more official "end of atmosphere" height where orbits would be 100% stable almost forever, but I'd imagine that orbits much lower would be stable for a very long time (by human standards).

[impwarhamer]

I don't think it's possible for a satelite to be in a 100% stable orbit.

[Motokid600]

Well you did a pretty good job of answering your own question. Yea technically theres traces of gases all the way out to the moon iirc. As for the question of how far out a satellite needs to orbit before air resistance no longer factors into its station keeping routine ( there's gravitational influences of course you gotta deal with ) I'm.. not too sure. 800+km up I'd say your satellite would take a human lifetime to deorbit. So anywhere beyond that point you just have to deal with the Moon and Sun's gravity.

[Camacha]

There is atomic gas througout a lot of space, so you will need to draw a line somewhere and call it quits. A defined edge is just not there.

[Cydonian Monk]

Technically, all of those numbers are correct at certain times of the year and under certain conditions. (Though the ISS orbit is a bit more nuanced, with apoapsis ~440km and periapsis ~300km.) There is no exact altitude where the atmosphere "ends", just like there is no exact altitude where the layers of the atmosphere transition. It depends on time of day, time of year, latitude, solar activity, local weather, all sorts of factors. For example, the Thermopause can range from ~400km at its lowest to ~1000+km at the highest, and as you observed that's not even the upper layer of the atmosphere.

The Exosphere is even more varied, and measured by matter that is in orbit of Earth or in a suborbital trajectory, not so much in its atmosphere. So the upper limit of the Exosphere would then be the point where the force of gravity, the matter's momentum, and the pressure from external factors (solar wind, gravity of other bodies, etc) all balance out [Edit: for atomic hydrogen]. Above that and external factors will pull the matter out of orbit. There really isn't a point where an orbit would be 100% stable, such as in the mathematically ideal 2-body physics of KSP.

So I suppose my question in response would be "For what definition of 'Atmosphere'?"

Edited July 4, 2015 by Cydonian Monk