[Source: NASA, Near Earth Object Program, full page: (LINK).]
Comet 2013 A1 (Siding Spring) will make a very close approach to Mars in October 2014
NASA/JPL Near-Earth Object Program Office / March 5, 2013
On Oct. 19, 2014, Comet 2013 A1 (Siding Spring) will pass extraordinarily close to Mars, almost certainly within 300,000 km of the planet and possibly much closer. Our current best estimate has it passing about 50,000 km from the surface of Mars.
This is about 2.5 times the distance of Mars' outermost satellite Deimos or less than twice the Earth close approach distance of 2012 DA14 on February 15, 2013.
Since the observation span available for orbit determination is still relatively short, the current orbit is quite uncertain and the nominal close approach distance will change as additional observations are included in future orbit estimates.
Currently, Mars lies directly within the range of possible paths for the comet and we can't exclude the possibility that the comet might impact Mars.
Our current estimate for the impact probability is less than one in six hundred and we expect that future observations will allow us to completely rule out a Mars impact.
Although the current heliocentric orbit is hyperbolic (i.e., eccentricity greater than one), the orbit is elliptic when expressed in the frame of the solar system's barycenter. After more than a million year journey, this comet is arriving from our solar system's distant Oort cloud. It could be complete with the volatile gases that short period comets often lack due to their frequent returns to the sun's neighborhood.
During the close Mars approach, the comet will likely achieve a total visual magnitude of zero or brighter as seen from Mars-based assets. The attached illustration shows the comet's approximate, apparent visual magnitude and its solar elongation angle as a function of time as seen from Mars. Because the comet's apparent magnitude is so uncertain, the brightness curve was cut off at apparent visual magnitude zero. However, the comet may get brighter than magnitude zero as seen from Mars. From Earth, the comet will not likely reach naked eye brightness but it could brighten to visual magnitude 8 as seen from the southern hemisphere in mid-September 2014.
Rob McNaught discovered Comet 2013 A1 Siding Spring on January 3, 2013 at Siding Spring Observatory in Australia. Pre-discovery observations located in the archives have extended the observation interval back to Oct. 4, 2012.