A cloud of debris spreading through low Earth orbit following the collision of two satellites poses a new risk to many scientific missions and may signal the demise of the Hubble Space Telescope. NASA is monitoring the increased threat carefully, and if it is as bad as some fear, the agency may have to cancel the proposed shuttle-servicing mission slated for later this year. Without that mission, the telescope's days are numbered, even if none of the new debris comes anywhere close to it.

At 04:56 GMT on 10 February an active communications satellite owned by Iridium Satellite of Bethesda, Maryland, and a defunct Russian military-communications satellite collided some 800 kilometres above Siberia at more than 10 kilometres per second. The cloud of debris initially consisted of 600 objects large enough to be tracked by the US space-surveillance network, and experts expect that number to grow to more than 1,000 within the coming weeks. Simulations suggest there will be millions more pieces too small to track.

A preliminary analysis by researchers at the University of Southampton in the United Kingdom shows that a head-on collision between the satellites would have released some 50 kilojoules of energy per gram, about ten times the yield of TNT and perhaps a hundred times more than the energy released in China's 2007 test of an anti-satellite weapon. That test also broke up a satellite and exacerbated the debris problem, which has already affected some satellites (see ). "It's totally unprecedented," says Graham Swinerd, a reader in astronautics at Southampton. If the blow was only a glancing one, though — if, for example, the Iridium satellite was snagged by a boom believed to have been deployed on the Russian satellite, rather than hitting the main body — the situation may not be so bad.

Iridium, which operates a constellation of 66 low Earth orbit satellites providing satellite-phone services, says that it regularly monitors data about space debris but had no prior warning of the collision. There has been no official comment from the Russian government.

Don't take the A-Train

The crash took place in a band of space heavily used by Earth-observing satellites, and space agencies are now closely monitoring the debris field as it spreads. NASA's 'A-Train' constellation and the Envisat mission of the European Space Agency (ESA) both orbit at very similar altitudes to that of the crash, and are thus at particular risk (see graphic). "Right now we are doing a statistical analysis of what the increase in collision probability will be," says Heiner Klinkrad, who heads ESA's space-debris office in Darmstadt, Germany.


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The increased risk to astronauts in the International Space Station seems to be "relatively low", according to Mark Matney, an orbital- debris specialist at the Johnson Space Center in Houston, Texas. But the collision puts in jeopardy a shuttle mission to repair the Hubble Space Telescope in May. The risk of impact for a mission to the space station is about 1 in 300, but for missions to the higher and more tilted orbit of Hubble the risk is greater. Even before last week's collision, the added debris from the 2007 Chinese test had pushed up the Hubble mission's risk of a catastrophic impact to 1 in 185. NASA's usual limit on such risk is 1 in 200, so Matney describes the situation before last week as already being "uncomfortably close to unacceptable levels". "This is only going to add on to that," he says. Matney believes that the agency will know within a week or two whether the mission can go ahead.

The blame game

In the immediate aftermath of the collision, experts were divided over whether Iridium should have seen it coming. "This should never have happened," says Geoffrey Forden, a space analyst at the Massachusetts Institute of Technology in Cambridge. Data provided by the US military showed that the two satellites would come within around half a kilometre of each other. "They should have manoeuvred Iridium away from it," he says.

But predicting satellite collisions is a tricky business, says Richard Crowther, head of the UK delegation to the UN Committee on the Peaceful Uses of Outer Space (UNCOPUOS). "It's actually a complex, time-consuming thing to do," he says. Even the best predictions are only probabilistic, and manoeuvring a spacecraft can involve costs and risks that outweigh the chance of collision.

Making avoidance easier is a matter of data and resources. Whereas Russia has its own system for tracking objects in space, the rest of the world depends more or less entirely on data released by the US Department of Defense's Space Surveillance Network. The military keeps far better analyses of its data than those released to the public, according to Brian Weeden, a former analyst with the US Strategic Command, which oversees the Pentagon's sensor network. These analyses are used to look out for dangers to military and intelligence satellites, as well as some high-value civilian missions such as the International Space Station. Those data should be released in the interest of "the public good", says Jonathan McDowell, an astronomer at Harvard University who keeps track of satellite launches as a hobby. McDowell believes that nations should consider a multi-national "space air-traffic control" to warn of such collisions.

More must also be done to prevent the further growth of orbital debris, adds David Wright, a researcher with the Union of Concerned Scientists, a non-profit group based in Cambridge, Massachusetts. At the moment, UNCOPUOS has a set of guidelines for limiting space debris, such as venting leftover propellant to prevent explosions. But Wright believes there should be binding rules for nations that launch into the cluttered region where the latest crash occurred. The debris created by each collision increases the risk of the next, and without action the number of incidents is bound to rise dramatically — possibly even leaving low Earth orbit completely unusable.

"I would like to see some sort of effort to make [existing guidelines] mandatory with some sort of enforcement mechanism," Wright says. Sanctions or fines should be imposed on negligent operators.

At the moment, the only legal framework for such action appears in the United Nations' 1967 Outer Space Treaty, which says that one nation can be liable for damage to another nation's satellite. But the wording of the treaty is too vague to be of much help in this case, according to McDowell. "Whose fault is it? Did one of them come up from behind?" he asks. "I don't think the road-traffic rules quite work." McDowell stops short of calling for an international treaty governing the low Earth orbit, but he does believe that "rules of the road" should be established to try and clear up the legal issues surrounding this and future collisions.

Liz DeCastro, a spokeswoman for Iridium, says she is unsure whether the company will pursue legal action against the Russian government.