Goddard Space Flight Center
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Is it possible for two orbiting satellites to collide?

Possible, certainly. The real question is, how likely such collisions are. Many things are possible (e.g. you getting hit by a falling meteorite) but are ignored as too unlikely.

Collisions between satellites are indeed unlikely, but their likelihood increases rapidly with the number of satellites: increase the number of satellites 10 times and, other things being equal, the likelihood of collision grows 100-fold.

It all depends on the orbits of course. Most satellites move in low-altitude Earth orbit, 600-1000 kilometer above the ground. At any times, this space is filled by thousands of pieces of matter--satellites, rocket stages, cast-off pieces of hardware (like weights used to slow down satellite spin), etc., about 100,000 pieces, most of them fragments from exploding rockets, but also including some 7500 larger accountable pieces of space hardware. Space is huge, but all these are moving rapidly. Luckily, all motions are essentially in the same direction (west to east, chosen to take advantage of the Earth's rotation) with almost the same speed. Even so, that speed is enormous, and collisions still may occur, since the orbits make different angles with the Earth's equator.

So far, the problem is not serious. One definite collision has been recorded in July 1996. The French satellite Cerise, launched in 1995, collided with debris from a 1986 launch, and broke off a stabilizing boom. In this case it was soon noted that the satellite had lost orientation and control was reestablished. Fine grains of debris occasionally hit the space shuttle, leaving impact marks in the heat tiles and even in the windows; to avoid damage to the sensitive front of the shuttle, at times when no reason exists to do otherwise, it flies tail-first. A "ding" 1/16 inch across, in a window of the shuttle, may be seen at http://satobs.org/image/sts-70_win.gif.

The main fear is that as the number of satellites grows, collisions will become more frequent. True, satellites in low orbits are also gradually removed , for air resistance makes them spiral downwards until they are burned up in the denser low atmosphere. But more collisions also produce fragments which increase the hazard.

High orbits, such as those of communication satellites which hang above fixed spots on the rotating Earth, can last millions of years.
Unless somehow removed, such satellites will stay up almost indefinitely, long after they have stopped giving useful service.

Dr. David Stern is a research physicist, retired from Goddard Space Flight Center after 40 years. He has created educational web pages about a wide range of topics in space, astronomy, physics and magnetism. A "focus page" with links to the main files and to 80+ subjects is at: http://www.phy6.org/prospect.htm

These widely used web sites have led to a large volume of e-mail questions, and web sites "Get a Straight Answer" list the more interesting answers. The largest of these, at: http://www.phy6.org/stargaze/StarFSubj.htm, has currently 121 items; the other two sites are linked from there. Have a look!