C/2013 A1 was discovered by Robert McNaught at the Siding Spring Observatory in New South Wales, Australia, on January 3. When the discovery was made, astronomers at the Catalina Sky Survey in Arizona looked back over their observations to find "prerecovery" images of the comet dating back to December 8, 2012. These observations placed the orbital trajectory of comet C/2013 A1 through Mars orbit on October 19, 2014.
The chance for a huge impact with the Red Planet is relatively low, for now. According to calculations by NASA's Jet Propulsion Laboratory (JPL), data suggests the comet is most likely to pass at 0.0007 AU (63,000 miles from the Martian surface).
But the comet has only been observed for 74 days, and the comet's track has a "cone of uncertainty" that includes planetary impact. Comet C/2013 A1 may fly past at a very safe distance of 0.008 AU (650,000 miles), or its orbital pass could put Mars directly in its path. At the time of Mars approach, the comet will be traveling at 35 miles per second (126,000 miles per hour). It's still uncertain how big comet C/2013 A1 is, but if it did hit, the impact could be a global event.
Even without impact, such a close pass will mean that should C3/2013 A1 become heated by solar radiation enough to erupt with a tail and coma around its nucleus, our Mars rovers and orbiting observation satellites will have a very close-up view of the event. It could end up being a much more impressive sight than Comet ISON's inner-solar system pass later this year.