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Where Satellites Go When They DieTo a higher plane.

By Daniel Engber
Posted Tuesday, April 26, 2005, at 6:43 PM ET

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On Tuesday a DirecTV satellite was launched into geosynchronous orbit from a platform in the Pacific Ocean. The company expects the new satellite to last for about 12 years. What happens when it stops working?

According to regulations from the Federal Communications Commission, any satellite in geosynchronous orbit—i.e., at an altitude of just under 36,000 kilometers—must be moved farther away from the Earth at the end of its useful life. A complicated formula determines how high the satellite must go to reach a suitable "disposal orbit," but most end up about 300 km above where they started.

Up to a dozen geosynchronous satellites go out of service every year, and there are now several hundred derelicts in the disposal orbit. They may drift slightly from that altitude under the influence of the sun and moon, but they won't interfere with the operational satellites below.

Unless they explode, of course. Any unused energy source material on an abandoned satellite can pose a significant danger, and many derelicts have blown up over the years. An exploding satellite in a disposal orbit can spray space debris down toward the operational satellites. To prevent such explosions, defunct satellites are supposed to "passivate" by discharging batteries, releasing compressed gases, or dumping propellant.

Of course, a satellite needs a bit of propellant to reach a disposal orbit, a fact that limits the life span of some satellites. Even if all of the electronics and moving parts on the DirecTV satellite are still functioning after 12 years, it will still need to be decommissioned before it runs out of fuel.

For satellites that operate at lower altitudes (including those used for surveillance and GPS), a trip up to disposal orbits may not be necessary. Orbits tend to decay over time, at a rate determined by the initial altitude. Anything above 2,000 km will take millennia to return to Earth, while satellites further down might take centuries or decades. A satellite that starts out in a low orbit can be maneuvered downward to ensure that it falls from space in a reasonable amount of time. NASA recommends that low-altitude derelicts be brought close enough to the Earth to return within 25 years. The satellite's operator could also decide that it would be more energy efficient to push it up to a disposal orbit above 2,000 km.

When a satellite falls out of orbit, between 10 percent and 40 percent of its mass survives re-entry. Hundreds of objects from orbit do hit the surface of the Earth at random locations every year, but no one has ever been reported injured or killed by falling space debris.

How dangerous is the debris that stays in space? Space agencies around the world keep track of any objects that are more than a few inches in diameter. (There are about 13,000 of these.) Even a small bit of debris could cause significant damage to a working satellite, although collisions are rare and only a handful have ever been observed. The most recent occurred on Jan. 17, when a part from a U.S. rocket launched in 1974 collided with a piece of a Chinese spacecraft from 2000. Three new chunks of debris were created by the crash.

Next question?

Explainer thanks Nicholas Johnson of the NASA Orbital Debris Program and William Ailor of The Center for Orbital and Re-entry Debris Studies.

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Daniel Engber is an associate editor at Slate. He can be reached at .
Photograph of artist's rendering of the satellite Aura by Agence France-Presse/NASA.
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