Will An Asteroid Hit Earth?

From: http://news.discovery.com/space/will-an-asteroid-hit-earth.html
Will An Asteroid Hit Earth?

By Robert Lamb | Fri Jul 23, 2010 03:27 PM ET

PS1_dome_skyThe Panoramic Survey Telescope & Rapid Response System (Pann-STARRS) scans the sky. (Image courtesy University of Hawaii)
If you happen to live in fear of asteroids, then here's a sobering fact for you: Our planet absorbs asteroid impacts like a truck grille eats bugs. Plus, according to NASA's Dr. Donald K. Yeomans, most of them aren’t worth getting bent out of shape over.

"On a daily basis, we're hit with basketball-sized objects, and Volkswagen-sized objects come in a few times a year," Yeomans says. "Fortunately, the limiting size for something that will actually do ground damage is about 30 meters [98 feet], and you'd expect something like that to come in every 200 years or so on average."

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Yeomans heads NASA's Near Earth Object (NEO) Program Office, which continues to spearhead the global effort to identify and track significantly large asteroids in near-Earth space. That directive includes rocks more than a kilometer (0.6 miles) across, which typically hit the Earth in million-year intervals. These asteroids are capable of causing global consequences. Larger objects, including the ten kilometer “dinosaur killer” that occurred 65 million years ago, are capable of plunging the globe into apocalyptic winters that last for years. Fortunately, these large object impacts are even less frequent, hitting Earth at intervals in the tens of millions of years.

"Asteroid size has this interesting distribution where there are very few of the big ones," says Lowell Observatory research scientist Bruce Koehn. "The smaller the size, the more there are."

Just think of a bag of potato chips that's been shaken and crushed in a cramped grocery bag. How many full-sized chips do you think you'll find? Outer space, however, is a lot larger than a bag of chips.

"We've discovered almost 90 percent of the total population of NEOs larger than a kilometer, and none of them are a threat," Yeomans says. "So the next step is to extend the survey down to 140 meters [459 feet], which Congress has asked NASA to handle. That will take larger, wider field telescopes, which hopefully will come on line within a few years. Actually one of them is already on line, the Pan-STARRS telescope on Maui."

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It's a situation where knowledge is power, and just knowing where the asteroids are has decreased Earths' estimated risk factor tremendously.

"When our project started in the late '90s, the risk of dying from an asteroid impact was approximately the same as dying from an airplane crash," says Koehn. "Twelve years of near-Earth asteroid surveys have reduced the risk simply by increasing our knowledge. So the risk of being hit by something we haven't found is 10 times less than it was."

Will an asteroid hit the Earth? Certainly. A small asteroid might be burning up in the atmosphere or touching down in the wilderness right now without any consequence. Meanwhile, the risk of catastrophic impact continues to grow ever fainter. Still, Yeomans admits that the NEO search will likely continue for the foreseeable future.

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"We'll never completely find every object that has an Earth-threatening trajectory," Yeomans says. "Some of them are quite small and difficult to find till they're almost upon us."

Plus as Bruce Koehn points out, our solar system's dance of asteroids is hardly set in stone.

"Orbits change," Koehn says. "They change due to interactions with major planets and large asteroids. They change because of [close approaches between asteroids] and for a variety of other reasons. So while we know whether most of these asteroids are coming near the Earth, we don't know the details of these interactions because we don't know the orbits well enough to calculate these close approaches. So there is kind of a fundamental level beyond which we cannot know, and we'll always have that level of risk."

Eyes on the Future
As scientists continue to watch the skies, it's important to realize that charting NEOs is far from merely a matter of planetary defense. They're also items of great scientific interest.

"The asteroids in the most Earth-like orbit are the ones that threaten us most, but they're also the ones that are easiest to get to," Yeomans says. "The Obama administration has said that the next human space flight target is likely an asteroid, so these near-Earth asteroids are not only a threat, but they're opportunities for exploration and future space resources."

Near earth asteroids may even unlock secrets about the origins of life on Earth, as asteroid likely brought life-necessitating water and carbon-based molecules to the early planet.

So if you've ever cowered under your covers at the thought of an asteroid impact, lighten up and try to focus on their scientific importance. Even if a real threat should present itself, scientists already have mitigation strategies lined up to deflect them.

"The risk is quite small," Koehn says. "It was never anything to lose sleep about, but it's even less now."