Based on just 74 days of observation, Comet C/2013 A1 (Siding Spring) is projected to miss the red planet by just 109,200km during a fly-by in October next year.
Given the level of uncertainty, this is mighty-close to an impact.
All it would take is the most minor of errors on orbital measurement to cause the estimate to gobble up that tiny miss, turning it into a collision.
Should there be a collision, an impact akin to that which decimated the dinosaurs on Earth will result. This may-well affect the pie-in-the-sky plans to send a married couple on a round-trip mission to Mars in 2018.