|(Last updated 05/11/2013. Click on image to enlarge.)|
The screenshot above was generated using the freeware space simulation Orbiter. It depicts Comet C/2013 A1, aka Siding Spring, during its closest approach to Mars on October 19, 2014.
Also used to generate the above screenshot was the Horizons Web Interface, maintained by the Jet Propulsion Laboratory at the California Institute of Technology. I generated the ephemeris with this web interface, and then plugged the numbers into an Orbiter scenario.
There's only an observational arc of 211 days - so there WILL be a really decent fly by - with enough refinement to the projected orbit (for now) to say it WILL NOT hit Mars.
Since C/2013 A1 is a hyperbolic comet and moves in a retrograde orbit, its velocity with respect to the planet will be very high, approximately 56 km/s. With the current estimate of the absolute magnitude of the nucleus M2 = 10.3, which might indicate the diameter from 10 to 50 km, the energy of impact might reach the equivalent of staggering 2×10¹º megatonnes! This kind of event can leave a crater 500 km across and 2 km deep. Such an event would overshadow even the famous bombardment of Jupiter by the disintegrated comet Shoemaker–Levy 9 in July 1994, which by some estimates was originally 15 km in diameter.Stay tuned, Dear Reader (if any). As new observations over time refine the orbit of this comet, I plan to update the screenshot and/or post a short video derived from an Orbiter scenario.
A short video: