On October 19, Comet 2013 A1 Siding Spring will nearly hit Mars as it passes 0.001 AU from the Red Planet. The nucleus of the comet is estimated to be around 4 kilometers wide, which would make quite an impact if it did collide with the Red Planet at 56 kilometers a second. The comet will come within 132,000 kilometers of Mars, which is close enough that the planet and comet’s atmospheres are expected to interact, possibly triggering auroras on Mars. A comet has never passed that close to Earth as far as history has recorded. If one did, it would be comparable to coming within one-third the distance of the moon. The orbit of Comet 2013 A1 Siding Spring should be altered by this close pass—look for more information on the comet’s new trajectory after it passes Mars (assuming it escapes the Red Planet’s gravity).

The comet is expected to make its closest approach at 18:28 UT on October 19, which is 7:30 in the evening in London, for example. Mars is the reddish “star” setting in the southwest in the constellation Ophiuchus, to the right of the teapot asterism of Sagittarius, for those looking from the Northern Hemisphere. In North America Mars will be above the southeastern horizon, however it will still be daylight.