By Marc Kaufman
Washington Post Staff Writer
Friday, March 9, 2007

The risk that an asteroid capable of wiping out humanity will crash into Earth is minuscule, new calculations suggest, but the chances of a smaller one destroying a city or setting off a catastrophic tsunami remain unclear and may be higher than previous estimates.

The calculations were presented at a four-day meeting in Washington this week, leading scores of scientists present to conclude that NASA needs to move aggressively to meet a congressional deadline for identifying most of the potentially hazardous smaller asteroids and to develop ways to deflect them if they home in on Earth.

But in a report released to Congress yesterday, the space agency said it does not have the funds to do the precautionary work, called for in its 2005 authorization bill.

The agency said it is technically feasible to meet the congressional goal of identifying most small "near Earth objects" by 2020, but it said it would have to rely on telescopes built for other purposes and on spacecraft being developed by other agencies. It did not address who would fund research on ways to destroy or divert an asteroid before it became a danger.

"Due to current budget constraints, NASA cannot initiate a new program at this time," said the report, obtained by The Washington Post.

The NASA document was immediately criticized by the chairman of the House Committee on Science and Technology, Rep. Bart Gordon (D-Tenn.).

"We are still reviewing the report, but it's clear that NASA's recommended approach isn't a credible plan to achieve the goal specified in the NASA Authorization Act," he said in a statement. "The Committee will continue to pursue this issue in the coming year with the goal of obtaining a more responsive approach."

The chairman of this week's Planetary Defense Conference, William Ailor of the Aerospace Corp., a nonprofit established by Congress to support the Air Force's space defense program, said scientists generally agree that the risk to Earth from large asteroids is small. Researchers have identified more than 700 of these potential "planet killers" -- out of an estimated 1,000 -- and found that not one is on a collision course with Earth.

"But with the smaller ones, the asteroids in the [150-yard] range, we're finding more and more," Ailor said yesterday. "They're hard to detect, and it's hard to predict where they are headed, but they can do a great deal of damage." NASA estimates that there are as many as 100,000 of the smaller asteroids in near-Earth orbit and that about 20 are "potentially hazardous."

The most recent significant asteroid to crash into Earth hit Tunguska, Siberia, in 1908. It exploded , with the force of a 15-megaton nuclear bomb and created a blast area 62 miles across.

Pete Worden, director of NASA's Ames Research Center and keynote speaker of the conference at George Washington University, said a similar asteroid landing on Washington would destroy the city and most of its suburbs. He said researchers have calculated the risk of "death by asteroid" to be about the same as dying in an airplane crash if you fly once a year. That calculation includes both the likelihood that the event will happen and the number of people who would be killed if it did.

Worden said it would probably cost about $1 billion to meet the congressional goal of identifying by 2020 90 percent of near-Earth asteroids 150 yards or more in diameter. "We know how to find objects most likely to be a problem," he said. "But we do not yet have congressional funding to move ahead."

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.