asteroid passing within 21,000 km of Mars and only 16,000 km from the moon Deimos. The pre-discovery observations were located by Andy Puckett in the archive of the Sloan Digital Sky Survey II at the Apache Point Observatory. 2 January 2008: NASA scientists revised the probability of an impact with Mars to 1-in-28 after more observations were reported by Bill Ryan with the 2.4-meter telescope at New Mexico Tech's Magdalena Ridge Observatory. The uncertainty region was reduced to roughly 200,000 km and still intersected Mars, but the most likely path moved a little farther away from the planet. 8 January 2008: NASA scientists revised the probability of an impact with Mars to 1-in-40 after refinements to the analysis of the Sloan precovery observations and observations with the 3.5 meter telescope at the Calar Alto Observatory in Spain. The uncertainty region was reduced by a factor of 3. 9 January 2008: Following several new observations, NASA reduced the uncertainty region and effectively ruled out a Mars collision. The chance of collision became only 1-in-10,000 (0.01%). The best estimate was that around 12:00 UTC the asteroid passed about 26,000 km (0.00017AU) from the planet's center (about 7 Mars radii from the surface). Analyses show there is no possibility of an impact with either Mars or Earth in the next century. This trend of increasing probability of impact followed by a dramatic decrease is typical as uncertainties are gradually reduced. In December 2004, a similar trend was observed with 99942 Apophis where the predicted probability of impact with Earth in 2029 at one point reached as high as 2.7%. Estimates of resulting impact If the asteroid had collided with Mars, it would have hit with a velocity of about 13.5 km/s (8.4 miles per second), and would have produced an explosion equivalent to about 3 megatons of TNT. Due to the thin atmosphere of Mars, it was predicted that the asteroid would have reached the surface intact and blasted out a crater approximately in diameter. A crater this size would be equal to the size of the Meteor Crater in Arizona, United States. NASA officials say if it had hit Mars, it would have done so north of the location of the Opportunity rover. is roughly the size of the cometary object that caused the Tunguska event in 1908, in remote central Siberia, Russia. Due to the Earth's greater gravity, an impact with