For example, to deflect a 700-foot asteroid, the astronauts envision a 20-ton nuclear-powered spacecraft shaped like a pendulum, with the heaviest part closest to the asteroid. This arrangement will produce the greatest gravitational force and will also help keep the ship stable as it hovers next to the asteroid.p>
But the first real-world test might involve a much bigger asteroid known as 99942 Apophis, which is between 1,000 and 1,300 feet long and weighs perhaps 50 million tons. There is a very small chance-from 1 in 5,000 to 1 in 10,000-that this massive rock will hit Earth in 2036. Love and Lu say that if a gravitational tractor were sent up before 2029, when the asteroid will make a close pass to Earth, the job would be relatively easy, requiring just a one-ton spacecraft to make the necessary small adjustment in Apophis´s trajectory. "It´s like changing the angle of a bank shot in pool," Love says. "A little change early on has a large effect down the line."