|Comet 2013 A1. Credit: NASA.|
NASA officially says that on 19 October 2014, Comet 2013 A1 (Siding Spring) will pass extremely close to Mars, almost certainly to 300,000 km from the planet, possibly much closer. The current accurate estimating is that it will come to about 50,000 km from the surface of Mars. This is about 2.5 times the distance of Mars's Deimos, farthest satellite of this planet, or more than twice the distance to Earth of the asteroid 2012 DA14 from 15 February 2013.
Since the available observation period is still relatively short, the current orbit determination, is quite uncertain and the distance of the approach will change after further observations to be included in estimates of future orbit.
Currently, Mars is in the range of possible direct path of the comet and we can not exclude the possibility of an impact with Mars. Estimating the probability of impact is more than 1 in 600, and we expect that future observations will allow us to completely eliminate an impact with Mars.
After more than a million years of travel, the comet comes from the Oort cloud of our Solar System. This comet could be complete, with volatile gases that on short period comets often missing because their frequent returns near the Sun.
During the approach to Mars, the comet will probably reach a total visual magnitude zero or brighter.