Potential Mars impactor update

I've been keeping my eye on the Minor Planets Mailing List for updates on the status of 2007 WD5, the asteroid that, as of Friday, had a 1 in 75 chance of hitting Mars. The Moon has been too bright to permit most observers to make any new observations, but there have been a couple of "precovery" observations of the asteroid added to the database, permitting better guesses at its future path. An object is "precovered" when an astronomer looks back at old photos, knowing the approximate path of a recently discovered faint object, and manages to find that faint object in pictures taken even before its discovery. The person making the precovery (the "precoverer?") gets no discovery credit for the object, but such observations help a lot in nailing down an object's past position and, therefore, the shape of its orbit.

On Sunday (December 23), one member of the Minor Planets Mailing List, Aldo Vitagliano, reported that three such precovery observations had been added to the Near Earth Objects Dynamic Site (NeoDys), which maintains a database of Near Earth Asteroids and their orbits; here's the NeoDys page for 2007 WD5, the potential Mars impactor. Vitagliano said that the precovery observations reduced the impact probability to around 0.3%, or less than 1 in 300. But then another member, Christian Kjaernet, replied today to say that the Jet Propulsion Laboratory's HORIZONS system, which provides highly detailed information on the orbits of (as of 8:15 Pacific time on December 27, 2007) "395,667 asteroids, 2,453 comets, 168 planetary satellites, 9 planets [guess HORIZONS hasn't demoted Pluto yet], the Sun, L1, L2, select spacecraft, and system barycenters," gives a Mars impact probability of 3.949%, or 1 in 25.

I'm not a regular user of either NeoDys or HORIZONS so I can't evaluate the primary information for myself; I'm just reporting secondhand news here. I'll keep you all posted with the best information I can find on 2007 WD5. Again, I'll emphasize that the most likely outcome of our attention to this object will be to find that it'll harmlessly pass well above Mars' surface; but such a close approach is exceedingly rare, and it'd be awfully exciting if it did hit!