1 Answer

Depends on the vehicle and speed.

The following is a quote:

340,000 feet or about 62 miles is generally the distance above the Earth it takes to reach space. That is the altitude that SpaceshipOne had to attain twice last year in order to win the X-prize.
However, this is not a "border" that you can cross where you have air on one side an space on the other. Our atmosphere really extends out into space well beyond the orbits of most satellites. It actually just gets thinner and thinner the higher you get. Most of our atmosphere is right down here where we live. When you fly in a commercial jet, I have heard from several sources that you are above half of the atmosphere. Even if this number is slightly off by a few thousands of feet, you can see that half of our atmosphere occupies the area up to 30 to 40 thousand feet above sea level and the other half goes up to thousands of miles. It gets so thin that it takes years and years of very slight drag to bring a satellite down low enough to burn up on re-entry at that 62 mile altitude or there abouts. Increased activity of the Sun will also make our atmosphere expand and contract which can cause a satellite to come down much faster than predidcted. That's what happened to our first space station, Skylab back in the late 1970's.

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