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The comet has a negligible 1 in 2000 chance of striking Mars and that probability seems to be decreasing as new data about the comet's orbit is gathered. See the vid at the first link.
NASA Sees Comet That Will Buzz Mars This Year
Comet Siding Spring will approach within 86,000 miles (138,000 kilometers) of Mars on Oct. 19, potentially putting on a good show for NASA's Red Planet spacecraft. But the dust shed by the comet as it barrels toward the sun may also endanger the agency's Mars orbiters, so officials are already mapping out possible risk-mitigation plans. NASA's Mars missions will join the observation campaign when Siding Spring gets closer to the Red Planet. For example, the Mars Reconnaissance Orbiter (MRO) may get good images of the comet's core during the close approach.