Anything falling through the atmosphere has what's called a terminal velocity. This is the speed at which the upward force, or drag, from air resistance equals the downward force of gravity. Typically, a falling object, like a skydiver, speeds up until it reaches terminal velocity. If something like the space shuttle starts out in the airless vacuum of space, it can reach a speed higher than its terminal velocity. But as soon as it starts passing through the atmosphere, friction will slow it down. This friction generates heat—about 3,000° for the space shuttle. Just how much heat depends on how fast the object is going, as well as its size, shape and mass.