The end of a spaceflight is the time when the astronauts aboard are exposed to the greatest danger. The spacecraft has to re-enter the Earth’s atmosphere from space. At this point it is traveling very fast and due to the Earth’s gravitation, the speed increases rapidly. As it rushes into the atmosphere, friction with the air heats the outside of the spacecraft which literally becomes a fireball. (See the illustration below.) Only the heat-resistant shield prevents it from burning up like a meteor. The astronauts have to manoeuvre the spacecraft in such a way that the heat shield strikes the air.
Besides, they have to be careful about the angle of re-entry. If the angle is too steep, the friction would be so much that the spacecraft may burn up in spite of having the protective shield. If the angle is too shallow, the spacecraft would behave like a flat-bottomed stone skipping on the lake’s surface and may bounce back into the space — never to return.