On Sunday, Oct. 19, 2014, at about 18:30 UTC (14:30 Eastern), Mars will experience a very close encounter with a comet.
The comet C/2013 A1 (Siding Spring) will pass just about 130,000 kilometers (80,000 miles) from the surface of Mars. There is no danger of an impact, but the planet will pass through part of the comet's tail (which is composed of gas molecules and dust).
NASA and other space agencies have taken precautions to make sure the spacecraft at Mars won't be hurt, but they're also hoping to capitalize on this unprecedented opportunity to see a comet VERY up close and personal. I'm not sure just when we'll start seeing data from them, but I highly recommend keeping an eye on Emily Lakdawalla's blog page and her Twitter feed. She is really great about staying up to date and relaying accurate information as soon as she has it.
I'll try to keep up as well, and if anything interesting happens I'll let you know. It's generally a good bet to follow me on Twitter too, as I'll be linking to things there, including news from other folks as it's confirmed.
Update, Oct. 18 at 15:30 UTC: The Virtual Telescope Network will be streaming images of the comet and Mars live starting at 16:45 UTC Sunday.
Update 2, Oct. 18 at 19:00 UTC: D'oh! I forgot to say: Also follow Karl Battams on Twitter, and keep your finger on the refresh button of the Coordinated Investigation of Comets page. Tons of info there!
Update 3, Oct. 19 at 18:00 UTC:ESA is holding a live webcast about the comet encounter. Due to technical difficulties, the ESA live 'cast has been moved to a Google Hangout.
A thought: The NASA comet page says the coma (the big fuzzy cloud of gas surrounding the solid nucleus of the comet) is about 20,000 km across. At closest approach, that means that if you were standing on Mars, the comet would appear to be over 8° across! That means that if you have a big hand, you could just barely block it with your upraised fist.
That's astonishing. What a view that would be! And while the astronomer part of my brain is envious and wishes we could see something like that from Earth, the human part of my brain is screaming obscenities at the astronomer part of my brain. In real life, it's probably best comets keep their distance from us.