Debris From Space Collision Poses Threat to Other Satellites

Scientists at NASA are keeping close tabs on two clouds of debris from Tuesday collision between U.S. and Russian satellites to determine how much of a risk they pose to the agency?s Earth-watching spacecraft and, possibly, the Hubble Space Telescope.

The rare collision between a U.S. Iridium 33 communications satellite and the defunct Russian military communications satellite Cosmos 2251 is unprecedented, marking the first time two intact satellites orbiting Earth have accidentally crashed into and obliterated one another, NASA officials said. Their smash-up created two large clouds of space debris that are currently being tracked by the U.S. Space Surveillance Network.

The debris poses a greater risk to science satellites than to the International Space Station, which is currently home to two American astronauts and a Russian cosmonaut, since the collision occurred 490 miles (790 km) above Siberia. The space station flies in an orbit about 220 miles (354 km) above Earth.

?This is like over 400 kilometers above the station, so we do believe that some of the debris is going down through station altitude. But it?s a very, very small minority of the debris clouds,? said Nicholas Johnson, chief scientist of NASA's Orbital Debris Program Office at the Johnson Space Center in Houston. ?For robotic spacecraft at higher altitudes, the answer?s a little bit different. So one by one we?ll be looking at those.?

Earth observations satellites, such as NASA?s Aqua and Aura spacecraft in orbit 438 miles (705 km) above Earth, are particularly vulnerable - though the risk of an impact is still low - and there?s another satellite in a 497-mile (800-km) orbit just above the impact level, Johnson told SPACE.com late Wednesday. The Hubble Space Telescope orbits the Earth at about 372 miles (600 km), Johnson said.

?That?s a little bit farther away, but it?s a lot bigger too. All that matters,? Johnson said of Hubble. ?It?s about how close you are to the debris cloud and how big you are.?

It will be weeks before the U.S. Space Surveillance Network pins down an accurate count of the number of individual debris pieces created in the event, but unofficial estimates put the damage somewhere in the 500 count. The U.S. Space Surveillance Network is currently tracking more than 18,000 separate man-made objects and debris at any given time, officials with the U.S. Strategic Command said Wednesday.

?This is the first time we?ve had two intact spacecraft collide, so it is a big deal,? Johnson said. ?But you know, it?s not unexpected.?

Johnson said that some satellites fly within a few hundred meters of each other every day. Each year, there are about six instances in which old satellites and satellite parts break apart in what scientists call ?fragmentation events.? Satellite components or spent rocket stages have accidentally collided three times before in the last 20 years.

In June 1997, an unmanned Russian Progress cargo ship slammed into Russia?s Mir Space Station, damaging a solar array and radiator, and punching a hole in the ship?s hull that depressurized one of its modules. Unlike Tuesday?s collision, that Progress spacecraft was deliberately heading for Mir, where it was expected to dock in a rendezvous system test.

?This was going to happen,? Johnson said of Tuesday's accidental collision. ?There was no doubt that it was going to happen.?

Johnson said the chances of a satellite being damaged by the debris from Tuesday?s collision are admittedly low, but as the collision itself proved, such things can happen. He does not expect to see reports of many secondary impacts from the event.

He pointed out that when China intentionally destroyed one of its aging weather satellites during a 2007 anti-satellite test, the impact created a cloud of more 2,500 pieces of debris.

?We don?t know if any of them have hit any other satellite, alive or dead,? Johnson said of the Chinese test debris. ?So the odds are still very small, but they?re bigger today than they were two days ago.?

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