EARTH’S ATMOSPHERE DIDN’T JUST STOP. IT PETERED OUT UNTIL IT became indistinguishable, by most measuring devices, from a perfect vacuum. Below about 160 kilometers of altitude, the air was still thick enough to rapidly drag down anything placed in orbit, so those altitudes were used only for short-term satellites like the early space capsules. The higher the altitude, the thinner the air and the more slowly orbits decayed.