Asteroids: How Often is Earth Hit by Space Debris?

Solar System debris rains down on Earth in vast quantities - more than a hundred tons of it a day. Most of it vaporizes in our atmosphere, leaving stunning trails of light we call shooting stars. More hazardous are the billions, likely trillions, of leftover rocks - comets and asteroids - that wander interplanetary space in search of targets.

Every few decades, on average, house-sized impactors collide with Earth. Typically they explode in the atmosphere, leaving no trace of a crater. Once in about a hundred million years, though, Earth is visited by an impactor capable of annihilating all life-forms that are bigger than a brief case.

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One killer asteroid we’ve been monitoring is Apophis, which is large enough to fill the Rose Bowl. On Friday the 13th, April 2029, it will dip below the altitude of our communication satellites. If its trajectory on that day passes within a narrow range of altitudes called the “keyhole,” then the influence of Earth’s gravity on its orbit will guarantee that seven years later, in 2036, on its next trip around the Sun, the asteroid will hit Earth directly, likely slamming into the Pacific Ocean. The tsunami it creates will devastate all the coastlines of the Pacific Rim. If Apophis misses the keyhole in 2029, we’ll have nothing to worry about in 2036.

Can we survive killer asteroids approaching Earth?

Yes, but it will not be easy.

Some people would like to blow potentially hazardous rocks out of the sky with a nuclear bomb. Others would deploy a radiation-intensive neutron bomb (the Cold War–era bomb that kills people but leaves buildings intact) to induce a recoil and alter the asteroid’s orbit. A kindler, gentler approach would be to nudge it into a different orbit with slow but steady rockets that have somehow been attached to one side — or with a solar sail, which harnesses the pressure of sunlight for its propulsion.
The odds-on favorite solution, however, is the gravitational tractor. This involves parking a probe in space near the killer asteroid. As their mutual gravity draws the probe to the asteroid, an array of retro rockets fires, instead causing the asteroid to draw toward the probe and off its collision course with Earth.