What causes the heat shield of the space shuttle to heat up so violently while re-entering Earth's atmosphere?

Question

Or any object for that matter (tiny pieces of asteroids that burn up before reaching us or the Apollo capsules)

Answer 1

Friction between the material of the heat shield or meteor, and molecules of air.

Answer 2

The sheer speed that the object is traveling and the molecules (oxygen, carbon, dust, ect.) rubbing/hitting against the object, acts like a sandblaster and breaks it down while heating it to high temperatures.

Answer 3

Technically it is the compression of the air ahead of the object moving at supersonic speed, as stated in the second answer. But friction is a good enough approximation to the true process for non-scientists. In the end, both processes turn energy of speed into energy of heat.

Answer 4

friction of the air molecules rubbing against it

Answer 5

Klingons shooting phasers at it.

Answer 6

Some people will say friction, but it is not friction.

The extreme compression put on the air as the shuttle (or any object) moves through it at very high speeds causes the air to become extremely hot and even ionize somewhat creating a hot plasma that will then in turn heat up the object passing through it.

Answer 7

It is the friction that an object experiences from the atmosphere around our planet.