US: Two Nasa astronauts have worked out a way to create a real-life version of a Star Wars "tractor beam" to keep an asteroid from crashing into the Earth.
Simply 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 doesn't 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" 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 such as 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 Deep Impact, suddenly became serious late last year when astronomers spotted an incoming asteroid whose probability of hitting Earth on April 13th, 2029, rose from one chance in 170 to one chance in 38.
By year's end it was clear that the 500m-wide (1,000ft) space rock, originally designated 2004 MN4 but now named 99942 Apophis, will miss - but only by 36,371km (22,600 miles). And if it gets exactly the right kind of gravity boost from the 2029 encounter, it will smack into the 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 Griffin, suggesting the agency send a mission to plant a radio transponder on Apophis to monitor its orbit better. Ruling out - or ruling in - a future impact requires the best available orbital data.
Schweickart heads the B612 Foundation, an organisation of experts that advocates the development of 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-Exupéry story.
Schweickart originally advocated a "tugboat" strategy of docking with an asteroid and pushing 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 ions, generated by an on-board nuclear reactor.
Once on station, the spacecraft would hover above the asteroid, using its engines to stay in place.
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 Deep Impact, the tractor is not messy.
"Impacts and explosions are difficult to predict and control," said Love. "When you're trying to save the Earth, you want them to be both controllable and predictable."
Unfortunately, Schweickart said, 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, it appears the Apophis threat can be handled with a conventional spacecraft.
In an October reply to Schweickart's June letter, Nasa associate administrator for science Mary Cleave outlined a potential response to Apophis. The critical task, she said, is to ensure that the asteroid does not pass through a 610m-"keyhole" (2,000ft) in space during its 2029 near miss.
Schweickart explained that the 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.