There's all kind of talk out there right now about how there's little would could do if we found an asteroid or comet coming at Earth that was going to do alot of damage. But I came across this the idea of a "Gravity Tractor" fascinating. Here's the basic idea from Wikipedia:
A gravity tractor (GT) is a spacecraft that deflects another object in space, typically a potentially hazardous asteroid that might impact Earth, without physically contacting it, using only its gravitational field to transmit the required impulse. The tractor spacecraft could either hover near the object being deflected or orbit near it. The concept has the advantage that essentially nothing need be known about the mechanical composition and structure of the asteroid in advance.
To be a little more specific in the case of an asteroid heading towards earth, the Gravity Tractor would push the asteroid just enough off track so that it would sort of bounce off the Earth's gravity. Here's more from the The Atlantic:
For any space object approaching a planet, there exists a “keyhole”—a patch in space where the planet’s gravity and the object’s momentum align, causing the asteroid or comet to hurtle toward the planet. Researchers have calculated the keyholes for a few space objects and found that they are tiny, only a few hundred meters across—pinpoints in the immensity of the solar system. You might think of a keyhole as the win-a-free-game opening on the 18th tee of a cheesy, incredibly elaborate miniature-golf course. All around the opening are rotating windmills, giants stomping their feet, dragons walking past, and other obstacles. If your golf ball hits the opening precisely, it will roll down a pipe for a hole in one. Miss by even a bit, and the ball caroms away.
Tiny alterations might be enough to deflect a space rock headed toward a keyhole. “The reason I am optimistic about stopping near-Earth-object impacts is that it looks like we won’t need to use fantastic levels of force,” Schweickart says. He envisions a “gravitational tractor,” a spacecraft weighing only a few tons—enough to have a slight gravitational field. If an asteroid’s movements were precisely understood, placing a gravitational tractor in exactly the right place should, ever so slowly, alter the rock’s course, because low levels of gravity from the tractor would tug at the asteroid. The rock’s course would change only by a minuscule amount, but it would miss the hole-in-one pipe to Earth.
This sounds fun, but it is incredibly difficult. First you have to find the asteroid or comet, then you have get spacecraft moving at up to 40 miles per second, and with great navigational precision. Plus, whatever you are using to propel the spacecraft could affect the asteroid/comet's path and negate the work the gravity of the vehicle is doing.
Gravity Tractors are one of several possible ways to deal with asteroids. None of them are that far into development and they all have serious technical challenges.
The comet picture is from WikiCommons.
