C/2013 A1 (Siding Spring) is scheduled for a very close flyby at only about 139 500 km (88 000 miles) away from the surface of Mars on October 19, 2014 around 18:27 UTC. That is less than half the distance between Earth and its moon and less than one-tenth the distance of any known comet flyby of Earth.
The comet is expected to flyby at about 56 kilometers per second (126 000 mph), relative to Mars, enveloping the Red Planet in its dusty coma as it goes by. It will not hit Mars, but questions still remain about the potential risks to the Mars spacecraft from the dust and particles released by the passing comet.
There are currently five spacecraft in Mars's orbit. One from India (Mars Orbiter Mission), one from the European Union (Mars Express) and three from the United States (MOD, MRO, MAVEN) + two Mars rovers (Curiosity and Opportunity). They will all try to take a close look at this historic flyby.
The period of greatest danger is when Mars comes closest (around 27 600 km) to the comet's trajectory, and not when the comet's nucleus comes closest to the planet. This is about 90 minutes after the closest app...
