An Asteroid Hurtles Toward Mars

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Scientists say that asteroid 2007 WD5 has a 1 in 75 chance of striking the planet Mars

Astronomers have good news, better news and some bad news about an asteroid known as 2007 WD5. The good news is that this 164-ft.-wide chunk of speeding space rock, discovered in November in an ongoing search for potential threats to Earth, won't hit our planet any time in the foreseeable future. The better news — for eager space-watchers — is that the asteroid, currently about halfway between Earth and Mars, has a plausible chance of hitting the Red Planet at the end of January. If it does, astronomers will be treated to an unprecedented sight.

The event itself, however, will have plenty of precedent. The craters that pock the surface of Mars, the Moon, Mercury and other Solar System bodies come from about four billion years' worth of this sort of thing. Earth has had plenty of collisions too; it's just that erosion, continental drift and vegetation have erased or hidden most of them. Not all, though: Meteor Crater, in Arizona, was blasted out some 50,000 years ago by an asteroid about the same size as 2007 WD5. A much bigger object, a few miles across, is thought by many scientists to be the reason the dinosaurs died out some 65 million years ago.

If 2007 WD5 does smack into Mars, every telescope on Earth will be pointed in that direction — just as they were in 1994 when Comet Shoemaker-Levy 9 crashed into Jupiter. In that case, the comet broke up while it was still in orbit, so astronomers watched nearly two dozen individual impacts. But Jupiter is made mostly of thick clouds, so there was no lasting scar, and because it lies so far from Earth, the event wasn't quite as spectacular as this one promises to be. Asteroid 2007 WD5 should release some 3 megatons of energy if it slams into solid ground near Mars' equator, and orbiting satellites will show the aftermath with crystal clarity.

Finally, the bad news: 2007 WD5 has only a 1-in-75 chance of actually hitting Mars, which means astronomers would be wise to be pessimistic. But the possibility of impact calls to mind a loosely related incident that occurred almost exactly 100 years ago, when something exploded above the Tunguska region of Siberia, flattening trees in a 25-mile radius, their trunks pointing outward from the epicenter of the blast. Scientists are pretty sure it was a comet or asteroid — about the same size as 2007 WD5, as it happens — that disintegrated from its own shock wave as it plowed through the atmosphere. (UFO enthusiasts have long been convinced it was a flying saucer that somehow made it across trillions of miles of interstellar space safely, only to blow up above Russia.) The scientific explanation would account for the aerial explosion, and also the fact that no crater has been found.

Except that now maybe it has. An Italian team has measured seismic waves reflecting off a high-density spot in the bottom of the suspiciously crater-shaped Lake Cheko, which lies close to the event's ground zero. It could be a piece of the original object — and finding it could help investigators understand exactly what happened a century ago.

If they find a burned-out flying-saucer engine, all bets are off.