MadSci Network: Physics

Re: Why do satelites eventually come down?

Date: Fri Jul 2 10:27:22 1999
Posted By: Adams Douglas, Senior Developer
Area of science: Physics
ID: 928974580.Ph

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:


Read about the LAGEOS satellites at:


Hope this answers your question,

-Adams Douglas
 Senior Developer
 Infonex, Inc.

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