A 1953 file photo shows the forests of Tunguska, Russia that were destroyed by a 1908 meteorite – and, 45 years later, still had not recovered. (AP Photo, File)

In 1908, an asteroid ripped through the Earth's atmosphere somewhere over Tunguska, a remote region of Russia's vast Siberian interior. Like the smaller meteor that sent a shock wave over Chelyabinsk on Friday, breaking apart catastrophically under its own weight and 10 kilometers per second speed, the Tunguska object appears to have exploded mid-air. The force of it leveled 500,000 acres of Siberian forests and may have been 1,000 times as powerful as the atomic bomb dropped over Hiroshima.

The odds of a similar object hitting a populated area are very low, but they are not zero. And the United States, like the rest of the world, is largely defenseless if that should happen.

Wired's Spencer Ackerman explains that "all the advanced air defenses that humanity has invested in" are "useless, useless" against meteorites. The really dangerous space debris, like the Tunguska object, are too heavy and move way too quickly to be simply shot down. Hitting a giant meteor with a missile is only going to change its trajectory or shatter it into pieces, which could make it even more dangerous. "Now you’ve got a shotgun blast instead of a single shot," a former U.S. Air Force Space Command office tells Ackerman.

Fortunately, there is an idea for a defense – an idea that Ackerman called a "gravity tractor" and has been around for at least a few years under similar names. It turns out that you don't need to detonate a nuclear warhead on an asteroid to change its course, you just need to place a heavy object nearby to divert its course.

If the gravity tractor ideas sounds crazy, it's only because Hollywood has been feeding us junk science on meteors for years now. "Many people think of a planet as a vacuum cleaner whose gravity sucks in everything in its vicinity," Gregg Easterbrook explained in a great 2008 story for The Atlantic. But it turns out that space objects are more likely to be a slingshot away from the Earth by its gravity; actually hitting the Earth requires the meteor to slip into a relatively tiny window, called a "keyhole." And knocking it away from that keyhole should be relatively easy. Here's Easterbrook:

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.

In other words, if the gravitational tractor works like it should, we don't have to hit the incoming meteor with a nuclear space missile, and we don't have to deploy a crew of tough-but-sentimental action heroes to the meteor's desolate surface. We just have to launch something heavy and unmanned to float next to the object long enough to sent it a fraction of a degree in a different direction so that it misses the "keyhole" approach.

But there is one piece of bad news. "When it comes to killer comets," Easterbrook writes, "you’ll just have to lose sleep over the possibility of their approach; there are no proposals for what to do about them." That threat, however remarkably minuscule, puts other international security problems, the vast majority of which are preventable, in a little bit of perspective, doesn't it?