Expert: Courtney Seligman - 9/6/2010

Question
Why does atmospheric friction increase the speed of a satellite in low Earth
orbit?

Answer
The direct effect of the friction is to slow down the satellite. But this makes the orbit "decay", or become smaller. As the orbit decays, the downward pull of gravity combines with the downward "motion" of the orbit to make the satellite go faster. (This is usually expressed as Kepler's Third Law, which states that smaller orbits have shorter orbital periods.)

In other words, the friction doesn't make the satellite go faster. But by making it go slower, it makes the orbit smaller. This leads to the apparently paradoxical result that the satellite actually ends up going faster.

This also works the same in reverse. If you could speed up the satellite (say, with a rocket thruster), it would move to a higher orbit. But as it did, the downward pull of gravity would slow it down, resulting in a longer orbital period.

This poses an interesting problem for spacecraft docking. If a spacecraft is very close to another one, and a little behind it, it uses its thrusters to go faster, so that it can catch up with the one in front, just as you might expect it to do. But this only works if they aren't very far apart, so they are in essentially the same orbit. If the trailing spacecraft is well behind the forward one, the way it catches up is to fire its thrusters in reverse and slow down. This drops it into a lower orbit, which takes less time to go around the Earth. Once the faster orbit puts it in front of the formerly forward rocket, the lower spacecraft uses forward thrusters to speed up, and move up and backwards, to finish the docking.