This graphic shows the predicted orbit of comet C/2013 A1 Siding Spring as it swings around the inner Solar System in 2014. On 19 October, the comet will pass very close to Mars. Its nucleus will miss Mars by about 132,000 km. As it flies by it, will be shedding material moving at over 50 km/s, relative to Mars and Mars-orbiting spacecraft. Even a tiny particle only 0.5 mm across moving at that speed could cause significant damage to a spacecraft.
NASA currently operates two Mars orbiters, and a third on its way, arriving in Martian orbit a month before the comet flyby. Teams operating those orbiters plan to adjust their orbits so that the spacecraft will be on the opposite side of the Mars when the comet is most likely to pass by.
Image Credit: NASA