Several weeks ago, I received a question from a reader to the effect of: are any of the surveys that turn up near-Earth asteroids also checking to see if there are any small bodies in the solar system on target for a Mars impact? I asked Steven Chesley, who I'd recently seen at DPS giving an interesting talk on communicating impact risks, and he told me that "as far as I know, nobody is looking specifically for Mars impactors. We, and a couple of other groups, check all NEAs [near-Earth asteroids] for close approaches to all planets and large asteroids, but there are many Mars-crossers that are not NEAs and these do not get checked. On the other hand, the non-NEA Mars-crossers that have been discovered so far will tend to the large side since these objects are necessarily farther from Earth, and thus fainter." I turned this in to a Q&A that I used on the November 5 Planetary Radio show, and a more detailed version wound up in the Questions and Answers column in the latest issue of The Planetary Report.
So it's quite a coincidence that on November 20, the Catalina Sky Survey spotted a new object that now bears the name 2007 WD5 on a course that passes near Earth. When observations from two other near-Earth asteroid monitoring facilities, Spacewatch (at Kitt Peak) and the Magdalena Ridge Observatory (in New Mexico), were used to flesh out the shape of WD5's orbit, it was discovered that although the asteroid doesn't pose a danger to Earth, it could impact Mars, and soon. Its orbit will pass within 50,000 kilometers of Mars on January 30. The predicted flyby is based upon a relatively small number of observations along a relatively short arc of its orbit, so there's a significant amount of uncertainty, enough that there is a nonzero probability (the current estimation is 1 in 75) that WD5's actual course intersects Mars on January 30.
WD5 is approximately 50 meters in diameter, so if it did hit, it would be a good-sized bang, producing a crater about a kilometer in diameter. (This is pretty close to the size of Victoria crater, Opportunity's haunt.) Chesley estimates that such a bang happens on Mars about once every thousand years.
I don't believe we would actually be able to see the flash from an impact from Earth. However, Chesley says that this size of an impact should create a dust plume that could be detected by one of the orbiters; even the rovers could detect a change in the dust component of the sky. And of course a 1-kilometer crater would be a pretty big target for HiRISE and CRISM on Mars Reconnaissance Orbiter -- in fact, that's quite big enough to be an interesting target for all the cameras on all the orbiters at Mars. You'd probably need to spot it first with one of the lower-resolution cameras, then zero in for a detailed look with one of the higher-resolution ones.
It's best not to get too excited, because the likelihood of an impact is pretty low. But it's certainly worth following up on to see if celestial mechanics will supply us with a natural experiment to study what happens when rocks slam in to planets. Unfortunately, from what Chesley says in a JPL podcast on the topic, it doesn't seem like this is a situation where amateurs will be able to do much to help the tracking effort; he says that "some of the biggest telescopes on the planet" will be required to do the followup. I expect I will be following up later this month to tell you that WD5 will miss Mars -- but I sure hope that WD5 turns out to have Mars' name on it!