"You see, the satellite transponder allows us to communicate with the spacecraft. Once we shut the transponder down, the satellite is 'dead' to us. There is no easy way of reviving it." says Guy Janin, Mission Operations Officer at ESOC, ESA's European Space Operations Centre in Germany.

One of these 'dead' satellites is Giotto, Europe's first deep-space mission, which measured the size and shape of Comet Halley in 1986. Giotto was finally switched off for the last time in 1992, having taken important observations of a second comet, Grigg-Skjellerup. "Giotto will most likely be cruising the Solar System forever," says Janin, "although it is possible that it may encounter the Earth again."

Other satellite ghosts are lucky and will not have to wait for the end of eternity to meet their maker. For example, ESA's Infrared Space Observatory, ISO, gave astronomers a very close view of the cool and hidden Universe for 3 years. Scientists expect ISO to reenter the Earth's atmosphere in 2014, which is relatively soon. On the other hand, the Hipparcos satellite will have to 'wait' until around 4500 before it comes to its fiery end.

Hipparcos was busy from 1989 to 1993 and pinpointed about 120,000 stars with astonishing accuracy during its lifetime.

Originally intended to live for just 3 years, the International Ultraviolet Explorer (IUE), simply refused to die. It carried on intercepting ultraviolet light from objects, such as comets and supernovae, every 90 minutes for more than 18 years. This makes it the most productive satellite in the history of space astronomy. Scientists finally turned off IUE in 1996, but it will fly on in a high orbit for millions of years to come.

None of these ghost satellites pose a threat to other space missions, since they are floating in relatively uncluttered high orbits. "We continue to monitor where these satellites are, but the possibility of a collision is so remote that we don't bother calculating their exact positions," says Janin. "If they were in Low-Earth orbit, which is very busy, that would be another story."

ESA archives all the data gathered by its missions, and this data is in constant demand from scientists all over the world. For example, Giotto's pioneering fly-bys of Comets Halley and Grigg-Skjellerup provided ESA scientists with invaluable experience. This is highly useful for a later mission, Rosetta, set for launch in early 2003. Rosetta will orbit Comet Wirtanen and drop a lander on it.

Dead satellites are doomed to wander through the never-ending expanse of space. However, the information they supplied during their golden years still continues to help us here on Earth.

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