Two NASA astronauts have figured out a way to create a real-life version of a "Star Wars" "tractor beam" to keep an asteroid from crashing into Earth.

By hovering nearby for perhaps a year, the astronauts say, the spacecraft's own gravity could minutely slow the asteroid's progress or speed it up, a process that 10 or 20 years later would cause the rogue rock to miss Earth by a comfortable margin.

"The beauty of this idea is that it's incredibly simple," said astrophysicist-astronaut Edward Lu. Since momentum does not dissipate in space, with enough time only a small early nudge is needed to cause a major orbital change.

Lu, who has made three trips to space, including a six-month stint aboard the international space station, and fellow astronaut Stanley Love, who has not yet flown, describe the design of their "gravitational tractor" today in the journal Nature.

The two are in the middle of a spirited debate among space buffs, astronomers and space agencies worldwide over what to do about "near-Earth objects" -- incoming comets and asteroids like the one that many scientists say caused the catastrophic "extinction event" that finished off the dinosaurs 65 million years ago.

This discussion, for years a sci-fi giggler among fans of movies such as "When Worlds Collide" and "Armageddon," suddenly became serious late last year when astronomers spotted an incoming asteroid whose probability of hitting Earth on April 13, 2029, rose from one chance in 170 to one chance in 38.

By year's end it was clear that the 1,000-foot-wide space rock, originally designated 2004 MN4 but now named 99942 Apophis, will miss -- but by only 22,600 miles. If it gets exactly the right kind of gravity boost from the 2029 encounter, it will smack into Earth seven years later with enough force to obliterate Texas or a couple of European countries.

With this in mind, former astronaut Russell Schweickart wrote a letter in June to NASA Administrator Michael D. Griffin suggesting the agency send a mission to plant a radio transponder on Apophis to better monitor its orbit. Ruling out -- or ruling in -- a future impact requires the best available orbital data.

Schweickart heads the B612 Foundation, an organization of experts who advocate developing a spacecraft that can alter an asteroid's speed enough to keep it from colliding with Earth. The foundation is named after the asteroid home of "The Little Prince" in the Antoine de Saint-Exupery story.

Schweickart originally advocated a "tugboat" strategy to dock with an asteroid and push it gently off its collision course, but he endorsed Lu and Love's idea as "a delightful way to pull an asteroid instead of pushing it -- we're all [in the foundation] sort of uncles to the tractor beam."

Lu and Love's design would use a relatively small 20-ton spacecraft powered by charged atomic particles called ions, generated by an onboard nuclear reactor. Such a propulsion system would -- at relatively low weight -- provide enough power to accelerate the probe to the speeds needed to run down the target asteroid.

With ordinary chemical fuel, "you'd be talking about a spacecraft that's 20 to 40 times larger," Lu said in a telephone interview from Houston's Johnson Space Center. "That kind of technology doesn't exist."

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.