Close comet flyby puts NASA's Mars spacecraft at risk

As a comet heads towards a close flyby of Mars Oct 19 this year, NASA is taking steps to protect its Mars orbiters while preserving opportunities to gather valuable scientific data, the US space agency said in a statement.

The "C/2013 A1 Siding Spring" comet's nucleus will miss Mars by about 132,000 km, shedding material hurtling at about 56 km per second, relative to Mars and Mars-orbiting spacecraft.

At that velocity, even the smallest particle - estimated to be about half a millimetre across - could cause significant damage to a spacecraft.

NASA currently operates two Mars orbiters, with a third on its way and expected to arrive in Martian orbit just a month before the comet flyby.

Teams operating the orbiters plan to have all spacecraft positioned on the opposite side of the Red Planet when the comet is most likely to pass by.

"Three expert teams have modeled this comet for NASA and provided forecasts for its flyby of Mars," said Richard Zurek, chief scientist for the Mars Exploration Programme at NASA's Jet Propulsion Laboratory in Pasadena, California.

"The hazard is not an impact of the comet nucleus, but the trail of debris coming from it," Zurek added.

However, he said that the hazard is not as great as first anticipated.

"Mars will be right at the edge of the debris cloud, so it might encounter some of the particles -- or it might not," Zurek said.

During the day's events, the smallest distance between Siding Spring's nucleus and Mars will be less than one-tenth the distance of any known previous earthly comet flyby.

The period of greatest risk to orbiting spacecraft will start about 90 minutes later and last about 20 minutes when Mars will come closest to the centre of the widening dust trail from the nucleus, NASA said.