A New Path for Asteroids

Once on station, the spacecraft would hover above the asteroid, using its engines to stay in place. Gravity "is a two-way street," noted Love, also speaking from Houston. Even as the spacecraft counters the asteroid's gravity, he said, its own gravity will pull the asteroid out of orbit.

"The velocity increment is small -- fractions of a centimeter [hundredths of an inch] per second," Lu added. "Suppose the asteroid is traveling 60,000 miles per hour. You want to make it 60,001." This, Lu suggested, might take a year or two years, but that would be enough, for the change would then accumulate over a decade or more, sending the asteroid harmlessly away. Bigger asteroids would take more time.

Unlike Schweickart's tug, the tractor would work even if the asteroid rotates or tumbles, and unlike nuking the asteroid -- Bruce Willis's solution in "Armageddon," the tractor is not messy.

"Impacts and explosions are difficult to predict and control," Love said. "When you're trying to save the Earth, you want them to be both controllable and predictable."

Unfortunately, Schweickart noted, research on nuclear-powered space vehicles has been cut dramatically to help fund President Bush's initiative to send humans to the moon and Mars. But fortunately, he added, it appears that the Apophis threat can be handled with a conventional spacecraft.

In an October reply to Schweickart's June letter, Mary L. Cleave, NASA's associate administrator for science, outlined a potential response to Apophis. The critical task, she said, is to ensure that the asteroid does not pass through a 2,000-foot "keyhole" in space during its 2029 near-miss.

Schweickart explained that Earth's gravity at close quarters will slingshot Apophis into a wider orbit, putting it in "resonance" with Earth -- the two bodies will meet up every sixth Apophis orbit and every seventh Earth orbit. If Apophis hits the keyhole in 2029, the result will be impact in 2036.

Cleave noted, however, that by analyzing Apophis's orbit during detailed observations next year and in 2013, scientists will have a much better idea of the asteroid's 2029 trajectory. If the threat still exists, a simple interception mission -- with chemical propellant -- would send a spacecraft in the early 2020s to smack into the asteroid like a celestial shotgun shell, changing its velocity by a few thousandths of an inch per second -- much more than enough to move it 2,000 feet by 2029.