Published online 15 April 2009 | Nature 458, 814 (2009) | doi:10.1038/458814b

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Collision debris increases risk to Earth-observing satellites

European study finds wreckage from recent collision in the spaceways.

The collision of two communications satellites on 10 February has significantly increased the risk to Europe's Earth-observing programme.

The European Space Agency's ERS-2 and Envisat missions are 30% more likely to face a catastrophic impact from space debris in the wake of the collision, according to Heiner Klinkrad, head of ESA's Space Debris Office in Darmstadt, Germany. The absolute risk remains small, but there were seven 'near misses' last year in which objects passed within 200 metres of the satellites. The satellites provide a range of environmental data, including in the case of Envisat some measurements of carbon-dioxide levels similar to, although less precise than, those that were expected from NASA's lost Orbiting Carbon Observatory (see 'NASA ponders 'carbon copy' of crashed mission').

The increased hazard is the outcome of a collision between a spacecraft in the Iridium satellite constellation and a defunct Russian military satellite (see Nature 457, 940; 2009).

At present, the debris cloud from the collision contains roughly 800 items of 10 centimetres or larger. Models show that number could rise to more than a thousand by the end of the month, says Richard Crowther, head of the United Kingdom's delegation to the United Nations Committee on the Peaceful Uses of Outer Space.

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The debris field is smaller and more concentrated than many had originally feared, according to Brian Weeden, a technical consultant with the Secure World Foundation, based in Superior, Colorado. That is probably because the two satellites dealt each other only a glancing blow. "It wasn't a dead-on collision," he says.

The concentrated debris field means that only satellites at fairly similar altitudes face a significantly enhanced risk. That could include some US and Canadian Earth-observing satellites. But spacecraft farther away, such as the Hubble Space Telescope and, when it services Hubble, the space shuttle Atlantis, seem relatively safe. US Space Command is tracking the larger pieces of debris, and ESA is making its own radar measurements, with the aim of arranging evasive action, if necessary, to avoid another accident. 

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