From Planetary Society Blog (Emily Lakdawalla)...
"WD5 most likely missed Mars, but we may never know"
Feb. 4, 2008 | 12:15 PST | 20:15 UTC
I just chatted with Steven Chesley of JPL's Near Earth Object program about the fate of 2007 WD5, the could-have-hit-Mars-but-most-likely-didn't asteroid. He said that based on the orbit predicted from the last observations of the asteroid, which were taken on January 9, they think it missed Mars by 6.5 Mars radii, which is a near miss in solar system terms but very far from the bull's eye if what you really wanted to see from the encounter was a new crater on Mars. (For those of you interested in the statistics, I asked him what the 1-sigma and 3-sigma uncertainties in the miss distance were; they were 1.8 and 2.2 Mars radii, respectively.)
Now, a 6.5-Mars-radius flyby is quite close enough to alter the course of WD5 significantly. Where's it going to go next? The answer is, no one knows. The uncertainty surrounding WD5's exact position with respect to Mars during the encounter hugely balloons when you try to figure out its future path. It essentially got a gravity assist from Mars, but without knowing exactly at what distance and, equally importantly, at what latitude it flew past the planet, we have no idea where Mars flung it. Mars isn't big enough to send an asteroid right out of the solar system (only Jupiter has strong enough gravity to pull off that trick), but a close flyby can really seriously change the orbit of a small body like WD5.
Link: Planetary Society Blog
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