NUDGE OF THE ASTEROIDS

Mechanical Engineering, Apr 2007 by Brown, Alan S

In late 2004, while most of the world was focused on the great tsunami, astronomers were watching another potential disaster. The asteroid Apophis, about one-third as wide as the span of the Golden Gate Bridge, was heading our way, and the odds of its slamming into the Earth had increased to one in 20 by December 27.

That night, additional observations led astronomers to recalculate an orbit with little likelihood of impact. When Apophis passes Earth again, in 2036, it will come within the orbits of two geosynchronous satellites. In a cosmos where millions of kilometers separate neighbors, this is a hairline miss.

At 415 meters, Apophis is an order of magnitude smaller than the asteroid that may have wiped out the dinosaurs when it hit Earth 65 million years ago. Yet it is an order of magnitude larger than the objects that formed a 1,200-meter-diameter crater in Arizona or flattened more than 2,000 square kilometers of trees above Russia's Tunguska River in 1908.

NASA estimated an impact of an Earth-Apophis collision equivalent to 880 megatons, four times greater than the Krakatoa volcano. "That's enough to destroy England or northern California," according to Steven Chesley, who as part of NASA's Near-Earth Object Office studies this sort of thing.

A close call like Apophis is supposed to be a once-in-15,000-years event, Chesley said. However, in 1972, a smaller asteroid skimmed Earth's atmosphere before skipping back into space. In 1989, an Apophis-size asteroid missed the Earth by six hours.

Humanity is not waiting helplessly for disaster. A former Apollo astronaut, Russell Schweickart, studied the issue. "We have the technical capability today to slightly reshape the solar system to enhance human survival," he said.

Survival starts with detection. NASA will complete a congressionally mandated survey of near-Earth objects greater than 1 kilometer by 2008. It plans to propose a survey of objects down to 140 meters.

If NASA finds an asteroid headed toward us, what then? One suggestion is to nuke it. A large nuclear weapon could vaporize a small asteroid. But it might only fracture a large asteroid into hundreds or thousands of large, dangerous, and hard-to-track missiles aimed at Earth.

Others suggest crashing a satellite into an asteroid to shove it off course. Done early enough, a small nudge could push an asteroid's trajectory millions of kilometers away from Earth.

Yet impacts pose problems. "Most asteroids are poorly held together collections of rocks, pebbles, and boulders," said another NASA astronaut, Edward Lu. "The energy of a 500-kilogram spacecraft moving at 5 kilometers per second is about 100 times greater than the gravitational energy binding Apophis together." An impact could fracture the asteroid, leaving large fragments headed toward Earth.

Instead, Lu and fellow astronaut Stanley Love propose a gravitv tractor. This is a small satellite that would use flared jets to hover above an asteroid. The gravitational attraction between the two bodies would give the asteroid the smallest of nudges into a safe orbit. Yet Lu readily admits that gravity tractors might not work with larger asteroids.

So how do we deal with an on-rushing asteroid? According to David Morrison, a senior scientist at NASA's Astrobiology Institute, "There's no consensus at all. There has been no developmental work on any of these ideas, and a consensus could take decades to develop."

Meanwhile, the clock is ticking. Even decades of advance notice offer little time to prepare. According to Morrison, it might take five years to develop a vehicle, five or more years to find a launch window to deliver it to the asteroid, and years or even decades for a gravity tractor to move a large asteroid.

Yet Morrison remains optimistic: "If we have 50 years of warning, people will find a way to build up our capabilities." ALAN S. BROWN