PHOTO: [left] JPL/NASA; [right] Fabrizio Bernardi, Marco Micheli and Dave Tholen/University of Hawaii Institute for Astronomy

22 January 2008—The NASA Near Earth Object (NEO) Program office in Pasadena, Calif., says that astronomical data from four different observatories now rule out a collision between a recently identified asteroid and the planet Mars. On 20 November 2007, astronomers working at the University of Arizona’s Catalina Sky Survey found the 50-meter asteroid. The discovery caused a scientific frenzy after early predictions indicated that the asteroid, designated 2007 WD5, would have a 1 in 75 chance of striking Mars on 30 January 2008. Scientists had hoped to study the crater the asteroid impact would have made on Mars and the geology it would have revealed.

The closest the asteroid will come to Mars is 26 000 kilometers, says Don Yeomans, an astronomer with the NEO office. It will pass by Mars’s two moons, Phobos and Deimos, missing them by 34 400 km and 20 300 km, respectively. “The impact probability for all three objects is now zero,” says Yeomans.

Observatories working within the NEO program routinely monitor the solar system for asteroids and comets that might eventually end up on a collision course with Earth. However, when one of these celestial wanderers is predicted to strike another planet or moon, it is a rare opportunity to obtain valuable scientific data on crater formation and other processes. “Knowledge of an upcoming impact with Mars allows plans to be made to observe the flash and dust cloud from the impact using ground-based telescopes as well as Earth- and Mars-orbiting spacecraft. It would have provided valuable information on the subsurface structure and composition of Mars,” says Yeomans. Had the asteroid actually hit Mars, the resulting dust cloud and crater could have been analyzed by the scientific instruments onboard the Mars Express, Mars Reconnaissance Orbiter, and Mars Odyssey spacecraft, all currently in orbit around the planet. The collision might even have been observed from the surface of Mars, as both of NASA’s Mars Exploration Rovers are still operational.

WD5 is the one of the smallest objects ever found near Mars and is the first asteroid to be considered a possible impact threat to that planet.