A gravity tractor, as envisioned by scientists, is a spacecraft that would hover over an asteroid on a collision course with Earth and, through gravitational attraction, accelerate or slow down the asteroid's rate of travel. By altering the speed, the gravity tractor could prevent the asteroid from striking Earth and wreaking environmental and economic havoc.
"It is possible to save the Earth from something like an apocalypse" with this kind of spacecraft, said Edward Lu, an astronaut and a scientist with NASA's Johnson Space Center, during a panel presentation on Friday in San Francisco. Lu was speaking at the annual meeting of the American Association for the Advancement of Science.
NASA's Edward Lu explains how a "gravity tractor" could save the Earth.
Strikes are extremely rare, but the results can be cataclysmic. In 1908, a meteor struck the deep woods in Siberia. It unleashed the energy equivalent of a 10- to 15-megaton atomic bomb.
The massive Meteor Crater in Arizona was caused by a meteor 40 meters across, said Lu.
Right now, there are 127 near-Earth objects (NEOs) that could possibly hit the Earth, the panel said. The most potentially dangerous is Apophis, a meteor 300 meters in diameter. Scientists now estimate that there is a 1 in 45,000 chance that Apophis will collide with the Earth in 2036.
Apophis on its own could not wipe out humanity, said Paul Slovic, founder and president of Decision Research. For that to happen, you would need a meteor with a 2-kilometer diameter. Still, it would be a mess.
"It would destroy England if it hit, or Northern California, but not the world," he said.
More potentially lethal asteroids will likely be turned up in the next 15 years as space agencies expand their survey for these objects, according to Russell Schweickart from the Association of Space Explorers. The odds for some of these objects could drop to 1 in 1,000 or even lower, he said.
"Extinction can be caused by a large asteroid impact. But with an early warning system we can prevent this. We can't prevent a tornado, but we can prevent this," Schweickart said. "And we prevent it by slightly reshaping the solar system."
The gravity tractor works on the slow-and-steady principle. The tractor would begin to hover several years or decades before the potential collision; with Apophis it could begin hovering as late as six years before impact. The tractor would then try to accelerate or decelerate the asteroid by very small amounts. Changing its speed would have the same effect as slowing down a car slightly: If accomplished early enough, collisions could be avoided because the intersection would be clear once the car arrived.
Others have proposed blowing up asteroids or trying to land on them and subsequently steer them. Blowing them up raises the problem of space debris, said Lu.
Blowing up or changing direction also can have unforeseen consequences. "There is a random element of deflection. You don't know what the results are going to be," said Lu.