Although air resistance is helpful in slowing down the rate at which spacecraft are pulled back to the Earth due to gravity, the friction generated by the spacecraft hurtling homeward results in extremely hot temperatures. When a spacecraft re-enters the Earth's atmosphere, it becomes hypersonic, or travels faster than the speed of sound [source: NASA]. The temperatures generated by this speed and the resulting friction can cause surface temperatures on the spacecraft to reach about 2,691 degrees Fahrenheit (1,477 degrees Celsius) [source: FAA].
There are a number of steps that a spacecraft passes through as its crew members prepare to re-enter the Earth's atmosphere after their space mission. First, the spacecraft turns around and flies backward to slow itself down from its high-velocity orbit. Orbital maneuvering engines push the spacecraft out of orbit and turn it toward Earth. As the spacecraft descends, it turns so that its blunt side faces down, reminiscent of a belly flop into a swimming pool. NASA has found that if a spacecraft come back to Earth with the blunt side facing down, a shock wave is formed, which keeps some of the heat away from the spacecraft. Another advantage to re-entering the Earth's atmosphere blunt-side first is that it further slows down the rate of descent.
The Apollo series of spacecraft were not designed to be reused. Part of the spacecraft was destroyed on re-entry, while the command module would land in the ocean and the crew would wait for pick-up. Shuttle craft, however, were built specifically to fly on more than one mission. The pilot of the space shuttle takes control of the computers once the shuttle is close enough to the Earth's surface and brings it down on a landing strip, similar to an airplane. The shuttle also uses a different type of heat shield -- made of special ceramic materials -- than those used on previous Russian and American missions. The tiles on shuttles are designed to burn slowly and conduct heat away from the astronauts.
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