|MadSci Network: Physics|
Blaine, most satellites in Low-Earth Orbit have their orbits decay due to atmospheric drag. Although the atmosphere is very thin at low orbital altitudes (100 - 500 miles) it is not absent and there is a very gentle drag slowing the satellite down. As it slows down its maximum altitude drops lower and it's exposed to denser atmosphere which slows it down more and so forth. A big satellite, like the Mir space station will slow down more than a small one because the largeer size of Mir runs into more thin atmosphere and creates more drag. Even so, most low-orbit satellites remain in orbit for years without fuel or orbit corrections--they just slowly orbit lower and lower. And if you go to higher orbits they can remain up for hundreds or even thousands of years. If they're high enough they won't come down at all. There is a satellite called Lageos which was launched in the 1970's. It is in orbit about 8,500 miles from Earth and was specifically designed to be very stable in orbit (it's small, very heavy and in a high orbit). It's not expected to decay for millions of years. In fact it has a picture inside of the way Earh's continents looked when it was launched and what it's expected they will look like when the orbit finally decays. That way, if anyone finds it after it falls in millions of years (and they know about continental drift) they will be able to tell when it was launched. You can read about the decay of a particular satellite (ASCA) at: ASCA Read about the LAGEOS satellites at: LAGEOS Hope this answers your question, -Adams Douglas Senior Developer Infonex, Inc.
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