October 19, 2014 might be a bad day for Mars. At least if Comet 2013 A1 (Siding Spring) has anything to do with it.
The latest trajectory of the comet generated by NASA’s Near-Earth Object Program Office indicates the comet will pass within 186,000 miles (300,000 kilometers) of Mars—and there is a strong possibility that it might pass much closer, maybe as near as 31,000 miles (50,000 kilometers). That’s only about two and a half times that of the orbit of Mars’ outermost moon, Deimos.
Scientists have been watching Siding Spring (named for Australia’s Siding Spring Observatory, where astronomer Rob McNaught first observed the comet) since October 2012. While further refinement to its orbit is expected, at the present time Mars lies within the range of possible paths for the comet. This means the possibility of an impact cannot be excluded. At the moment chances are less than one in 600 that this will happen. Astronomers figure that future observations will probably completely rule out a Mars impact. But still...
The comet will be a spectacular sight in the sky of Mars during its closest approach. It will achieve a magnitude of zero, about the same as the star Vega in earth’s sky, or even brighter. (From our planet, the comet is not expected to reach naked eye brightness, but it may become bright enough [about magnitude 8] that it could be viewed from the southern hemisphere in mid-September 2014, using binoculars, or small telescopes.)
The view from Mars
Scientists at the Near-Earth Object Program Office estimate that comet Siding Spring is a refugee from the distant Oort cloud. It may take as long as a million years to orbit the sun. So seldom has it come this close to the sun that it may still have the volatile gases that short period comets often lack. This makes it a nearly pristine sampling of the primeval material from which our solar system was formed.