A recently discovered asteroid which passed close to the Earth in November, is now headed towards a very close passage by Mars in late January, and there is a small chance that it could hit that planet. The probability of a collision is only 1 chance in 75, but scientists are excited about the possibility. If it happens, the impact would occur on January 30, 2008 at around 10:55 UT (2:55 a.m. PST).
In the likely event that the asteroid misses Mars, it could come back to the vicinity of the Earth years or decades later, but our routine hazard monitoring shows that there is no threat of an impact with the Earth.
Designated 2007 WD5, the asteroid was discovered on Nov. 20, 2007 by the NASA-funded Catalina Sky Survey using a 1.5m telescope on Mt. Lemmon, near Tucson. The object had already passed within 7.5 million km (5 million miles) of the Earth on Nov. 1, before it was discovered. Based on its magnitude, we estimate the asteroid to be about 50 meters (160 feet) across. As the accompanying diagram shows, it has already reached the halfway point between Earth and Mars. When it closes in on Mars, it will approach from the day side, and would then be very difficult to observe from any of the spacecraft on or around Mars. Our current best estimate predicts the asteroid will miss Mars by 50,000 km, but the miss distance is highly uncertain because the asteroid’s path is not known with sufficient accuracy. The uncertainty region during the Mars encounter currently extends over a million kilometers (700,000 miles) along a very slender ellipsoid only 1200 km (700 miles) wide, but the ellipsoid does intersect Mars. The zone of potential impact on the surface of Mars is approximately 800 km wide, and sweeps across the Martian equator from southwest to northeast, crossing the equator at roughly 30 deg W longitude. The MER Opportunity rover is close to the southern edge of this possible impact zone but clearly outside it.
The asteroid is becoming increasingly difficult to observe, since it is receding from the Earth and the waxing Moon is approaching the same part of the sky. But it should become observable again early in January. These new measurements will lead to a significant improvement in the orbit accuracy, and we will then be able to refine the probability that the asteroid might collide with Mars.
If the asteroid is indeed on a collision course, it would hit Mars with a velocity of about 13.5 km/s (8.4 miles per second), and would produce an explosion equivalent to about 3 MT of TNT. We can only speculate as to the effects of such an impact, but it would be reasonable to expect a crater nearly a kilometer across and a significant amount of dust lifted into the atmosphere.
An impact would not be unprecedented: 21 fragments of Comet Shoemaker-Levy 9 impacted Jupiter in July, 1994. Those impacts were predicted with near certainty almost a year before the impact. But, with a 1-in-75 chance, this asteroid’s possible impact with Mars is far from certain.