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A comet hurtling towards Mars is set to put three Nasa spacecraft at risk.

Nasa is taking steps to protect its Mars orbiters, while preserving opportunities to gather valuable scientific data, as Comet C/2013 A1 Siding Spring heads toward a close flyby on 19 October.

While the comet itself poses little danger, debris that is travelling in its wake could pose a problem if any spacecraft pass through its tail. 

Nasa is taking steps to protect three spacecraft that will be in orbit around Mars when Comet Sliding Spring makes a pass by the planet on 19 October (illustration shown). The comet will miss Mars by about 82,000 miles (132,000 kilometers), and its trail of dust particles could pose a threat to the three probes

The comet's nucleus will miss Mars by about 82,000 miles (132,000 kilometers), shedding material hurtling at about 35 miles (56 kilometers) per second relative to Mars and Mars-orbiting spacecraft. 

At that velocity, even the smallest particle - estimated to be about 0.02 inches (0.5 millimetres) 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,' explained Dr Richard Zurek, chief scientist for the Mars Exploration Program 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. 

'Using constraints provided by Earth-based observations, the modeling results indicate 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.'

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 center of the widening dust trail from the nucleus.

Nasa's Mars Reconnaissance Orbiter (MRO) made one orbit-adjustment manoeuvre on 2 July as part of the process of re-positioning the spacecraft for the 19 October event.

An additional manoeuvre is planned for 27 August.

The team operating Nasa's Mars Odyssey orbiter, meanwhile, is planning a similar manoeuvre on 5 August to get their spacecraft out of harm's way.

Nasa's Mars Atmosphere and Volatile Evolution (Maven) spacecraft, meanwhile, is on its way to the Red Planet and will enter orbit on 21 September.

Nasa's Maven orbiter (illustration shown) is due to join the Mars Reconnaissance Orbiter and Mars Odyssey in orbit around the Red Planet on 21 September. The team running the probe are already preparing a manoeuvre to ensure the spacecraft is not damaged by the incoming Comet Sliding Spring

The Maven team is planning to conduct a precautionary manoeuvre on 9 October, prior to the start of the mission's main science phase in early November.

Of course there won't just be operational spacecraft in orbit around Mars when the comet approaches; two active rovers are also on the surface. 

However Mars' atmosphere, though much thinner than Earth's, is thick enough that Nasa does not anticipate any hazard to the Opportunity and Curiosity rovers on the planet's surface, even if dust particles from the comet hit the atmosphere and form into meteors.

Rover cameras may be used to observe the comet before the flyby, and to monitor the atmosphere for meteors while the comet's dust trail is closest to the planet.

Observations from Earth-based and space telescopes provided data used for modeling to make predictions about Siding Spring's Mars flyby, which were in turn used for planning protective manoeuvres.