A tug in time would save Earth just fine

The concept is similar to an earlier idea of sending a space tugboat to push the asteroid off course—except the gravity-well spaceship won’t have to touch the asteroid to make it veer off course.

Hovering a few hundred feet above the asteroid, the spaceship would use the force of gravity as a tow line to pull the asteroid off course just enough to miss Earth. “It’s a very small force,” Lu said. “But over time it builds up.”

Lu and colleague Stanley Love, an astronaut at the Johnson Space Centre, calculate it would require a 20-ton “gravity tractor” a year of tugging to safely deflect an asteroid more than 600 feet across.The “tractor” would need about 20 years advanced notice to intercept the asteroid at a great enough distance from Earth so that only a small deflection would be needed.