When you walk down the street, you may feel a breeze against your face, but as the wind picks up to hurricane speeds, you can barely stand up and it feels like you are being pummeled by some unseen boxing gloves. Although the atmosphere of the Earth is pretty tenuous a few hundred miles up, at the speeds traveled by orbiting spacecraft of 20,000 miles per hour, the atmosphere takes on a substantially higher density as perceived by the spacecraft. The re-entry angle has to be chosen very carefully so that the spacecraft does not 'skip' off the atmosphere like a stone tossed at a body of water. This will happen if the angle is too flat for a given spacecraft speed. On the other hand, if the re-entry angle is too steep, the friction of the atmosphere against the spacecraft will heat the spacecraft up faster than its heat shield can dissipate the heat, and the spacecraft will burn up. So, for any spacecraft velocity and intended landing site on the surface, there is literally a window in space that the spacecraft must shoot through to be at the correct re-entry angle.