Friday, December 21, 2007

Is Mars in the crosshairs?

Image credit: NASA/JPL

Okay, I said I was done writing for the year, and I thought I was. But that was before this story broke, suggesting that Mars may be hit by a "big asteroid" at the end of January.

First, let's clear up that, as asteroids go, this rock is not "big." Yes, the asteroid is estimated to be about 160 feet (50 meters) across. That is a big rock by our standards, but tiny by asteroid standards. If it were to hit a city on Earth, that city would be severely damaged and a lot of people could be killed. But this is much, much smaller than the asteroid that wiped out the dinosaurs, and would not threaten our civilization.

Still, we'd like to know if Earth is going to be hit by anything big enough to endanger people, so there are several programs looking for these rocks. And, in November, they discovered this previously-unknown asteroid. Until we know its orbit better, it will be known by the boring catalog name of 2007 WD5. Since the asteroid can come close to Earth, it was put on lists to be closely watched.

When an asteroid is first discovered, we tend not to know its orbit very well. It takes months, or even years, of observations to be able to calculate its orbit well enough to know if or when it might hit a planet. As time goes on and more observations are made, the "error box," or uncertainty in the orbit, gets smaller. This is why you may hear from time to time that a newly-discovered asteroid has a 1-in-10,000 chance of hitting the Earth in a few decades, and a few months later, the chance drops to 1-in-10 million or even less.

The first orbits for the asteroid gave it a 1-in-350 chance of hitting Mars in late January. This was a pretty good chance, but most astronomers expected the chances to get much smaller when new data came in. So, when the new data actually increased the chances to 1-in-75, this became exciting.

Still, the odds are that the rock will miss Mars. We'll know more in the next few weeks, as more data come in and are analyzed.

If the asteroid does hit Mars, this is a chance for some useful science. We have only been able to estimate how large of a crater a rock of a given size will make; with a direct hit on Mars, we can measure it. This would let us know how much of a danger small asteroids like this are to the Earth, in the event we ever saw one coming for us.

Small asteroids hit the Earth and Mars all the time. Every meteor you see is a space rock burning up in our atmosphere. Every few years you'll hear of meteorites hitting somebody's house or car -- these are typically baseball-sized rocks. The bigger ones, like 2007WD5, probably hit every 100 to 1000 years.

In fact, an asteroid the size of 2007 WD5 is probably what caused the Tunguska Event of 1908, where the mid-air explosion of a small asteroid or comet leveled hundreds of square miles of Siberian forest.

So, in short, Mars probably won't be hit, but the chances of a hit are much higher than we've ever seen from an asteroid in modern times. And an impact would teach us a lot about small asteroid impacts, which could help us protect ourselves if we ever find an asteroid inbound toward Earth. For more details, see the press release from NASA's Jet Propulsion Laboratory

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