You must have witnessed in many movies that when a spaceship crashes or enters earth’s atmosphere it catches fire. But if you are thinking it is some kind of special effect then you are wrong. It is a true fact that the spacecrafts get hot while returning back to earth. And the reason behind this interesting phenomenon is explained below.
Friction, due to high speed is the reason why spacecraft heats up. Air molecules constitute earth’s atmosphere and the density is much lower in the space then in the lower region of the atmosphere. There is absolutely no concentration of air molecule in the space, so nothing hits or touches the body of the spacecraft even when it is spinning very fast at its orbital velocity. When the spacecraft enters the atmosphere filled with moving air molecules it starts hitting them. Since the spacecraft is moving at very high speed it hits each molecule faster and that causes friction. The air molecule density is quite high near the earth’s surface.
Every single molecule of air hits the spacecraft and use small bits of energy from the spacecraft. When a car or truck hits something on the road, then it might stop at some point but spacecraft has a lot of inertia from the high speed, so it is difficult to stop it, resulting in high levels of friction.
The velocity of the spacecraft slowly converts into heat energy and the deeper it goes into the atmosphere, the more the amount of heat energy is generated, and sometimes without proper heat resistance it might be possible that the spacecraft catches fire.
Hence, scientists have created many ways to slow down the speed of the spacecraft and also installed special ablative heat shield on the bottom of spacecraft to absorb maximum amount of heat generated.