Correction to This Article
A March 9 article about the possible threat to Earth from asteroids incorrectly described how a proposed "gravity tractor" would change the object's trajectory. It would not orbit the asteroid but would hover near it and deflect its direction by means of its gravitational pull. Also, a map with the article showing the possible path of the Apophis asteroid should have credited the B612 Foundation for the information.
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'Planet Killer' Not in the Stars, Asteroid Research Indicates

An asteroid struck Tunguska, Siberia, in 1908, exploding with the force of a nuclear bomb and leveling a large area.
An asteroid struck Tunguska, Siberia, in 1908, exploding with the force of a nuclear bomb and leveling a large area. (Associated Press)

NASA's asteroid-tracking is being done under the "Safeguard Survey" program, which is receiving $4.1 million a year until 2012 to find problem asteroids. NASA officials and scientists at the conference said most smaller asteroids cannot be spotted with current telescope technology. Two telescope projects under development by other agencies and by organizations in Chile and Hawaii could provide high-powered help to find such asteroids, but both projects lack NASA participation and secure funding.

To meet the congressional goal, researchers said, NASA would have to build a similar telescope of its own or launch a satellite observatory that could watch Earth from an orbit near Venus.

One of the major recommendations of the Planetary Defense Conference is to reconsider plans to close the National Science Foundation's Arecibo radio telescope in Puerto Rico. The scientists said the large instrument is especially useful in identifying the orbits of asteroids closest to Earth and that its loss would be a blow to asteroid-tracking efforts.

Both the NASA report and the conference tackled the technically complicated and emotionally charged question of how to deflect an asteroid. NASA concludes that the most effective way would be detonating a nuclear bomb nearby -- a tactic the agency rated 10 to 100 times more effective than other possibilities.

But sending a nuclear device into space carries risks, which led to NASA's conclusion that firing an unarmed missile to physically knock the asteroid off course is the most "mature" technology. It also described as potentially promising the concept of a "gravity tractor" -- a satellite that would orbit near the asteroid and would, through its gravitational pull, change the asteroid's trajectory.

NASA has no plans to test any of these possibilities, but the European Space Agency has been working since 2000 on an asteroid-deflection project called the "Don Quijote" mission. Ian Carnelli, of ESA's advanced concepts team, said the plan includes launching two spacecraft at an asteroid -- one to measure orbit and physical characteristics, and a second to hit it. Carnelli said ESA members will decide next year whether they want to fund the first stage of the project.

Most asteroids are in a belt between Mars and Jupiter, but some are knocked out of that region and head toward Earth's orbit. Asteroids, which were discovered about 200 years ago, are generally either solid rock or loose piles of rubble. Part of NASA's mission is to learn more about their physical characteristics so they can be deflected or destroyed, if necessary.

The United States sent a space probe to the large asteroid Eros in 2001. Japan landed a probe on the small asteroid Itokawa in 2005.

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