Where Space Begin

So far, science has not firmly defined where the Earth's atmosphere ends and space begins. A new study puts that boundary at 72 miles out, based on the fact that that's where charged particles from outside begins to overwhelm the air molecules in the wispy upper atmosphere.

Astronaut status, however, is given to anyone who reaches 50 miles out. For purposes of shuttle flights, NASA defines space beginning at 76 miles. The internationally recognized legal definition is 62 miles.

This is probably one definition that can reasonably vary depending on why the definition is required.