A commercial satellite collided with a Russian satellite over Siberia yesterday, yielding a cloud of fragments, according to a NASA scientist tracking space debris. The collision between the commercial satellite, belonging to the American communications firm Iridium, and the Russian satellite, believed to be defunct based on its advanced age, was the first of its kind, says Nicholas Johnson, chief scientist at the NASA Orbital Debris Program at Johnson Space Center in Houston. (A spokesperson for Iridium said a statement on the incident would be released shortly.)*

"In the past almost 20 years, there have been three other accidental collisions between objects in orbit, but they've all been very minor," Johnson says. "The most debris ever produced in an event was like four debris, and this is two intact spacecraft colliding, and we have hundreds of debris out there. We don't know exactly how many yet."

According to Johnson, the military sky-watchers who track satellites in orbit picked up the collision 490 miles (790 kilometers) above Earth Tuesday. "One of the things that they discovered yesterday afternoon ... was all of a sudden, where two satellites used to be, there were two clouds of debris," he says. The actual crash appears to have occurred just minutes before noon, Eastern Standard Time.

Johnson says NASA has already determined that the debris cloud poses "no significant new risk to the International Space Station." The next space shuttle mission, which may launch as early as February 22, should be in the clear as well, according to the space agency.

Such a collision between two intact spacecraft may be unprecedented, but it is not completely unexpected. "There are no rules of the road in space," Johnson says. "Anybody can fly anywhere they want." Even concerted efforts to track and guide spacecraft in orbit are subject to some uncertainty in trajectory estimates. At seven miles (11 kilometers) per second, Johnson says, "a little error means a lot."

*UPDATE (7:10 P.M.): Iridium's statement confirms the loss of the satellite, calling the crash an "extremely unusual, very low-probability event." The company says its network of 66 satellites plus in-orbit spares "is uniquely designed to withstand such an event, and the company is taking the necessary steps to replace the lost satellite with one of its in-orbit spare satellites."

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