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The sky is falling: Scientists confront asteroid threats

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NASA studied composition of this comet, Tempel 1, during Deep Impact, a 2005 space mission. (courtesy of NASA/ CNS)

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The Barringer Meteorite Crater in Arizona is nearly a mile wide and 570 feet deep. ***note small file size 1,000 x 750*** (courtesy of PDPhoto.org/ CNS)

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The red arcs mark the trail of an asteroid in space. (courtesy of NASA/ CNS)

To David Morrison, a senior scientist at NASA, the Earth orbits the sun in a sort of cosmic shooting gallery. Over 1 million asteroids spin around the sun, and it is Morrison’s job to figure out which of these bodies of rock, dust and metal could come crashing down on Earth.

Right now, NASA is tracking 127 asteroids that have a very small chance of striking the planet. That number is about to get a lot higher. Stronger telescopes, and a new mandate from Congress, will allow scientists to detect thousands of smaller asteroids more likely to hit Earth. And scientists are plotting ways to stop them, from "gravity tractors" to solar ray guns.

“There is no question that these will hit the earth,” said Russell Schweickart, a former Apollo astronaut who is involved in a group studying asteroids. “The question is how often we will have to do something about it.” In fact, Schweickart thinks world leaders might have to do something about it very soon, within the next 15 years.

In early March, Russian, European, Japanese and American scientists held a Planetary Defense Conference in Washington to discuss the threats and plot a strategy for dealing with them.

Identifying asteroids close to the Earth is the priority right now, said Dave Jonta, a conference spokesman. A large asteroid could cause what scientists call an “impact winter”: a huge volume of dust gets thrown into the atmosphere, completely or partially blocking the sun’s light, causing crop loss, disease and possible global starvation. And smaller asteroids could kill hundred of thousands, if not millions of people, Schweickart said.

NASA has identified most asteroids near the Earth that are 1 kilometer in diameter or larger, but the U.S. Congress recently ordered the agency to find and track smaller asteroids, too, those 460 feet in diameter and up. Only one in 50 of those asteroids have been identified.

To locate them, scientists use powerful telescopes that take detailed pictures of the night sky. A computer finds the moving objects against the millions of background stars in the pictures. NASA will need new, more powerful telescopes to find the small ones.

Because of the difficulty in projecting an asteroid’s orbit, scientists often can only predict the probability that a specific asteroid will hit the Earth. So international leaders might have to take action before knowing for certain what path an asteroid might take. "We may have to spend $300 million to fly a mission that in the end wasn’t needed," Schweickart said, "but that’s a lot better than living with a 10 percent chance that New York City will be hit."

Still to be determined is which country will be responsible for flying a mission to deflect an asteroid, and who will finance such a project.

To confront questions like these, a group of experts, scientists, diplomats and international lawyers will meet in May. The group will present proposals to the United Nations in 2009, for eventual ratification. “Then when something happens, we have a process,” Schweickart said.

Even smaller asteroids can be devastating. Tungusta, an asteroid that hit near Siberia in 1908, was only about 50 yards in diameter--half a football field wide. But it set off an explosion in the atmosphere that flattened and burned 200 square miles of Siberian forest.

And scientists estimate an asteroid the size of Tungusta or larger will hit the planet every 300 to 800 years. If world leaders decide to take action against every asteroid with a one in 10 chance of striking the earth, they may have to make decisions on asteroids the size of Tungusta or larger as frequently as once every 30 to 80 years.

At the Planetary Defense Conference, scientists discussed several ways to deflect an asteroid spinning toward Earth. "This is the first time in the planet's history we have the capability to defend ourselves against what we will know will happen," said William Ailor, chairman of the conference.

One option is to deflect the asteroid off course by slowly pushing it. This could be done by focusing a concentrated beam of sunlight on it, boiling off material to slow it down.

Scientists could also use a gravity tractor, a robotic spacecraft parked close to the asteroid. Because the two objects have gravitational force between them, the gravity of the hovering spacecraft would pull the asteroid, gradually shifting its course. “The weak gravitational force acts as a tow rope,” Schweickart said.

A high-speed spacecraft could strike the asteroid like a billiard ball. Or, scientists could set off an explosive nearby to push the asteroid out of its Earth-bound trajectory.

None of these methods have been tested. Schweickart is part of a group of scientists who would like to set up such tests, but so far no government has agreed to cover the cost of the project.

Morrison is hopeful about scientists' ability to spot and deflect asteroids. And he maintains a sense of humor about the issue.

“Since 1989 I’ve been working on this issue,” he quipped, “and I’ve been 100 percent successful. Not one person has died from an asteroid.”

E-mail: amt2139@columbia.edu