The intense heat caused by friction is also a problem in space travel. If you rub your hand on your forehead, you will feel this kind of heat. Once a spaceship is in outer space, there is no friction because there is no air to press against. But when the spaceship return to earth, it must go through air again. At first the air is very thin. But the closer the ship comes to earth, the denser the air it meets. A spaceship entering the earth’s atmosphere at full speed would get so hot that it would burn up and completely disappear.