A atmosphere is gasses and they just get thinner and thinner as you go out from a planet. Yet there must be some line they draw as to where the atmosphere ends. Like take earth or Jupiter. 1) What is it? And the trouble with Jupiter is it has only a small solid metal hydrogen core or something and the rest is slush and then gas. 2) How do they define “Jupiter”? Earth is just defined as the solid or liquid (ocean)part and Venus, Mercury, Mars, and Pluto are just the solid part. But then when they get out to the gas planets they count the gas as the planet, again raising the whole issue of where does the line come when they decide oh this area out from Jupiter is so rare that now we’ll call that space?
Well, for purposes of such equivalents as “what would you weigh on Uranus?” they usually choose as the “surface” of the gas giants the radius where atmospheric pressure is equal to that of Earth’s at sea level.
As for where Earth’s atmosphere ends, any official radius would be an arbitrary choice, but I know the space shuttle rarely orbits higher than 150 miles or so, and that height is considered to be above the atmosphere.
This really is a question for The Bad Astronomer, though.
Defining where “space” begins can be a very arbitrary line. The air force defined it as fifty miles so military X-15 pilots who exceeded that altitude were awarded astronaut wings.
Another common definition of a planet is that it’s the visible part. You can see more of Jupiter than just the metallic hydrogen (or whatever it is) core, so we call the whole thing “Jupiter”. When this standard is applied to stars (such as the Sun), it’s called the photosphere.
I believe the ICAO or whatever the body is that keeps the records defines (yes, fairly arbitrarily) the edge of space as being 100 km (62+ miles) above sea level. In practice, the atmosphere of Earth just keeps getting thinner and thinner, and is still measurable hundreds of miles up. The STS is decidedly within the atmosphere - I saw a long-exposure picture which showed a glowing halo around the STS’ tail while in orbit. The practical ‘edge of the atmosphere’ would be the shock wave where the solar wind is deflected around the planet, but this is determined largely by the magnetic field and has little to do with the gases in the vicinity. Also, this makes the edge of the atmosphere much closer on the sunward side, as the shock wave is teardrop-shaped.
The space shuttle undergoes a phenomenon known as “Shuttle Glow” (see http://sspp.gsfc.nasa.gov/hh/glo/glo.html for details of an experiment on shuttle glow).
It is caused by atomic oxygen in the atmosphere reacting with the shuttle’s surfaces. This effect has been noticed at heights of over 200 miles.
The height of an atmosphere is entirely arbitraty. But it is absolutely over at the magnetopause because at that point the solar wind takes over.