Hardy 6-tonne satellite zooms towards Earth

Hit the deck! A 6-tonne, abandoned NASA satellite with some particularly hardy parts is zooming towards Earth.

The Upper Atmosphere Research Satellite (UARS) was launched in 1991 to study chemical changes in the Earth's upper atmosphere and decommissioned in 2005. NASA says it will probably re-enter Earth's atmosphere on 23 September, give or take a day.

"This is the largest NASA satellite to come back uncontrolled for quite a while," says Nick Johnson, chief scientist for NASA's Orbital Debris Program Office at the Johnson Space Center in Houston, Texas.

Most of the satellite's mass should burn up but the agency has identified 26 parts likely to survive, including a 150-kilogram instrument mount and three 50-kilogram batteries. In total, more than half a tonne of debris should crash on Earth in a debris footprint some 700 kilometres long.

Cow flat

Based on the number of pieces the satellite is expected to break into, the total area it could potentially hit and the number of people in those areas, NASA estimates that it has a risk of approximately 1 in 3200 of injuring somebody (pdf).

That's well above the risk of 1 in 10,000 that NASA now requires before it launches a satellite, but UARS was designed and launched before NASA drew up those rules.

Johnson points out that no one has ever been seriously injured by space debris, although there are anecdotal reports that a cow was killed by debris from the Skylab space station, which re-entered over Australia in 1979.

Hugh Lewis, an expert in space-debris modelling at the University of Southampton, UK, agrees that the risk is low. "You have to remember that over 20,000 objects larger than 10 centimetres wide have re-entered the atmosphere since the beginning of the space age - and there have been no reported injuries from these events," he says. "At 1 in 3200, the probability of the surviving pieces of UARS causing injury are remote"

Legal unknown

If people or property were to be hit by the UARS debris they would chart new legal territory, says Joanne Wheeler, a specialist in space law at CMS Cameron McKenna in London, since neither the UN Outer Space Treaty nor the UN Convention on International Liability for Damage Caused by Space Objects have ever been tested in court.

NASA has not yet said where UARS will hit, other than somewhere between 57 degrees north and 57 degrees south. That area spans northern Canada through to the southern tip of South America and most of Europe and Asia and so encompasses most of the world's 7 billion population, along with a huge swathe of ocean.

Any wannabe space-junk collectors, be warned. On its website, NASA says: "If you find something you think may be a piece of UARS, do not touch it. Contact a local law enforcement official for assistance."