November 12, 2005 in Nation/World

Astronauts propose defense against Earth-bound asteroids

Guy Gugliotta Washington Post
 

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 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,” astrophysicist-astronaut Edward Lu said. 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” this week 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 like “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 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 only by 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 in June wrote a letter to NASA Administrator Michael 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 St.-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 on-board 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 mph. You want to make it 60,001.” This, Lu suggested, might take one 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 simply take more time.


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