Unfortunately, and in spite of the hype we are giving it, comet C/2013 A1 (Siding Spring) is not going to be a spectacular object in the night sky. Faint, and distant from Earth, only experienced observers with moderately sized telescopes have a reasonable chance of viewing it. Well, that is in our night sky, at least...
Mars: dry, desolate, dusty, and frigidly cold. It's very pretty to look at but not really the kind of place you go for a vacation. I'm sure the views would get tiresome after a while, and that whole suffocating and freezing to death thing would put a downer on most people's getaway. That said, if you are truly desperate to get a good view of Comet Siding Spring in October, there's is absolutely no better planet in the entire universe than Mars!
On October 19, 2014, comet Siding Spring will pass just 138,000 kilometers (86,000 miles) from Mars. On planetary scales, that's close... really close! It's about one-third of the average distance from the Moon to Earth, and many times closer than any comet has passed by Earth in at least the past several centuries. If we had a comet come that close to Earth today, it would be nothing short of breathtaking. (Important to note: even at that distance, it would not be any threat to Earth.) Alas, we get no such event any time soon (that we know of) at Earth, but we do have a pretty good idea what the comet will look from the surface Mars.
The reason we can see comets at all is because all the dust, ices and gas released from the comet's nucleus reflect sunlight. In general, the a bigger and/or more active comet will release more material, and thus will appear brighter. Also, the closer you are to a comet, the brighter it will appear. Finally the angle at which you're viewing it relative to Sun will also play a big part in its brightness. A comet directly in front of the Sun, for example, will be very much enhanced by "forward scattering" of sunlight. So we always try to factor these variables into predicting how bright a comet will be, but it is very much dependent on your own location relative to the comet. This brings me nicely back to Mars and Siding Spring.
We've established that the comet will be very close to the planet, so even though the scattering geometry won't be ideal, Comet Siding Spring is going to appear to be insanely bright from the Martian surface. Predictions hover around magnitude -6, which is brighter than Venus appears from Earth (-4.8). Even the spectacular comet Hale-Bopp in 1997 only topped out near magnitude 0 or so. So whichever way you shake it, Siding Spring will be an awe-inspiring sight from Mars. Just a shame we can't watch, right? Well, maybe we can...
We don't have people on Mars but we do have a couple of robots, and these robots have cameras that can - and occasionally do - look at the night sky. We heard in our first Siding Spring Workshop that the Mars rover teams hope to attempt night time observations of the comet. However, they made a point of making no promises. Mars is absurdly cold during the day, and even colder at night. Operating a rover in those temperature requires a lot of power, and that means very careful consideration of how and when to move cameras and other mechanisms on the rovers. The safety of the hardware is far more important than pretty pictures of comets. An additional complication is that the comet will be in twilight skies much of the time (Mars has a very extended twilight due to all the dust), so that will limit visibility of the comet until it's high in the dark night sky. But, that said, my understanding is that the teams are going give it their best shot, and we can all hope they are able to obtain the first ever images of a comet from the surface of another planet!
Anyway, all of this brings me to the purpose of this post. While we can't place ourselves on the surface of Mars to see the comet and watch it arc across the Martian night sky, we can simulate the path it will follow as if we were standing where the Mars Curiosity Rover is operating. And that is exactly what the awesome folks at INOVE have done! They're experts at producing fabulous interactive solar system models with their "Solar System Scope" application, and have produced great orbit models for Comet ISON and ESA's Rosetta mission, among others. So, while corresponding with them a couple of weeks ago, I tossed out the suggestion that maybe they could simulate the view of the comet as seen by Curiosity on the Martian surface. The response I got was very positive, and they absolutely came through! Check out this awesome interactive tool!
How cool is that?! Click on the "To Orbit" button to see the orbital view of the comet, and "To Mars" to see the view from Mars. It's a lot of fun to play with, and gives you a great feel for how relatively fast the comet will move through the Martian sky - almost as if you were standing there. I love it!
It is important to note that the image of the comet they use in there is just a generic simulation of a comet. We really don't know exactly what the comet will look like from Mars (hopefully the rovers will tell us that!), so that image is simply to illustrate the position of the comet in the sky. Regardless, it's a great tool to play with and maybe show off to some kids or non-astronomers, and spread the excitement of this once-in-a-lifetime cosmic encounter that we're all about to witness.