Astronomers are debating what to do about Earth's close encounter with an asteroid in 2029 and again in 2036 passages that might be too close for comfort.
Apophis, a 1,059-foot-wide asteroid, has excited astronomers since it was spotted last year. After observing it for a while, scientists concluded that it has only a 1-in-8,000 chance of ever smacking into Earth. But even that slim chance has them talking and NASA pondering how to keep track of it just in case.

"The most likely turn of events is that it will miss us," says Steve Chesley of NASA's Jet Propulsion Laboratory in Pasadena, Calif., which has monitored the asteroid since December as part of its normal watch over "near-Earth" asteroids. "We are prepared for the worst but certainly don't want to act too hastily."

In June, former astronaut Russell Schweickart petitioned NASA chief Michael Griffin to consider placing a transmitter on Apophis, which is named after an ancient Egyptian god of darkness and destruction, by 2013. Chesley says NASA will respond in a few weeks.

The key question about Apophis is whether its 2029 trajectory will go through a roughly 2,000-foot-wide region called a "keyhole," says astronomer Clark Chapman of Southwest Research Institute in Boulder, Colo. If it passes through that region that one-in-8,000 chance its course would be deflected to make an impact with Earth in 2036 very likely, he says.

This asteroid passes near Earth every seven or eight years, but the 2029 trajectory is expected to be its closest approach. In the petition, Schweickart warned that waiting for better estimates of Apophis' likely path in 2020, after another flyby of Earth, would leave little time to deflect the asteroid away from the keyhole.

If the asteroid were to strike Earth, scientists say the impact would be felt somewhere on a line stretching from Japan to the Caribbean. It would create a roughly 2-mile-wide crater and possibly create a tsunami.

But Chesley says directing the asteroid away from Earth might take only a blast the size of NASA's "Deep Impact" mission, which smacked a probe into the comet called Tempel 1 in July. A 2029 deflection mission would be easier because an asteroid practically saunters compared with the speed of a comet, and this one passes within 22,600 miles of Earth's center -"extraordinarily close for an object of this size," he says.

The asteroid should be visible, about as bright as the stars in the Big Dipper constellation, to observers in Europe, Chapman says. Such a close asteroid flyby comes only once every 1,500 years. "Certainly this is a profound scientific opportunity," Chesley adds.

Astronomers already are pondering what the 2029 encounter will do to the asteroid. The Earth's gravity will exert strong tidal forces on Apophis, which could change the asteroid's shape if it is only a loose agglomeration of rock. The flyby also will permanently alter the asteroid's orbit, although how is still uncertain.

Radar observations next spring should improve estimates of the asteroid's path and might indicate a lower impact risk.

"But there are more asteroids out there, so we should be paying attention," Chapman says.