When astronauts return from the International Space Station, their capsule hits the atmosphere at a speed of more than 17,000 miles per hour.
When they look out the window, this is what they see:
The sparks are the result of the huge amount of heat (roughly 3,000°F) that builds up on the surface of the Soyuz capsule due to the compression of atmospheric gases it causes as it descends. (The astronauts are protected from all this heat by a heat shield that faces the Earth as the capsule descends)
Soon afterward, a series of parachutes and landing engines deploy, slowing the craft down to a speed of about three miles per hour by the time it lands on the steppes of Kazakhstan.
Correction: This article originally said the capsule traveled at 500 miles per hour. It eventually slows down to this speed, but is going much faster upon initial impact. It also said that friction, not compression, was responsible for the heating, when in fact friction causes just a bit of the heat.