The possibility of a collision between Mars and an approaching asteroid has been effectively ruled out, according to scientists watching the space rock. Tracking measurements of asteroid 2007 WD5 taken from four observatories have greatly reduced uncertainties about its Jan. 30 close approach to Mars so that the odds of impact have dropped to 1 in 10,000, the Near-Earth Object Program at NASA’s Jet Propulsion Laboratory said in a posting on its Web site Thursday.