http://neo.jpl.nasa.gov/news/news153.html

Mars Impact Probability Increases to 4 Percent
Don Yeomans, Paul Chodas and Steve Chesley
NASA/JPL Near-Earth Object Program Office
December 28, 2007

The impact probability for a collision of asteroid 2007 WD5 with Mars on
January 30 has increased from 1.3% to 3.9%.

Pre-discovery observations of asteroid 2007 WD5, taken on November 8,
2007 have allowed its orbit to be refined and the uncertainties for the
late January Mars encounter have been improved. The impact probability
resulting from the recent orbit refinement has increased to a surprising
3.9% (about 1 in 25 odds). The uncertainty region during the Mars
encounter now extends over 400,000 km along a very narrow ellipsoid that
is only 600 km wide. Since the uncertainty region intersects Mars
itself, a Mars impact is still possible. However, the most likely
scenario is that additional observations of the asteroid will allow the
uncertainty region to shrink so that a Mars impact is ruled out. In the
unlikely event of an impact, the time would be 2008 January 30 at 10:56
UT (2:56 a.m. PST) with an uncertainty of a few minutes.

The pre-discovery observations were located by Andy Puckett, a recent
Ph.D. from the University of Chicago who has since moved to the
University of Alaska Anchorage. Dr. Puckett located the observations in
the archive of the Sloan Digital Sky Survey II, which contains extensive
repeat coverage of 300 square degrees along the sky's celestial equator.
The observations were taken using a 2.5 meter aperture telescope at the
Apache Point Observatory near Cloudcroft, New Mexico. For the recent
orbit refinement, these pre-discovery observations on November 8 were
added to the existing observations provided by the Catalina Sky Survey
and Spacewatch observatories (both near Tucson AZ) as well as New Mexico
Tech's Magdalena Ridge Observatory.