Trajectory of asteroid "2011 MD" on June 27, 2011, from the direction of the sun.
We wish we could be more reassuring about the whole asteroid-collision-wiping-out-all-life-on-Earth thing, but no can do. Just ask the dinosaurs.
The pipsqueak pebble that flew by Earth today, missing us by a 7,600-mi. (12,000 km) whisker (in space, that’s tiny) was never going to do us much damage. First of all, multiple tracking systems operated by NASA and the private sector had already plotted its course and knew that it would give us a pass. What’s more, its small size—just five to 20 meters (16 to 64 ft.)—guaranteed that even if it did hit us, it would burn up in the atmosphere long before it reached the ground. About 100 tons of interplanetary dust does descend through our atmosphere and reach the surface of the planet every day, but it’s utterly harmless.
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That’s the good news. The bad news is that the solar system remains a shooting gallery—not the way it was 4.6 billion years ago, when the planets were just forming and debris was everywhere, waiting to be gravitationally gathered in by whatever accreting planet it happened to pass. But still, space rocks abound—and not all are small enough to be eaten by the atmosphere.
NASA estimates that about once every 100 years, a space rock larger than 50 meters (164 ft.) should be expected to reach Earth, causing local disasters or tidal waves. Every few hundred thousand years, an asteroid of 1 km (.62 mi.) could strike, causing the kind of global devastation that wiped out the dinosaurs 65 million years ago. Asteroids are so dangerous because they don’t hit like a bullet fired at close range, leaving a clean hole wherever they strike. Instead, they come bearing all of the energy they accumulated in their high-speed flight and release it on impact. (To be precise, bullets do that too, but they’re so small and their flight is so brief the energy release is not as significant.)
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On June 16, 1908, an incoming asteroid thought to have measured from 5 to 10 km (3 to 6 mi.) burst in the air while still in descent above the Tunguska River in Moscow. The blast is thought to have been 1,000 times more powerful than the bomb dropped over Hiroshima. Mercifully, the area was only sparsely populated, but up to 80 million trees across 830 sq. mi. (2,150 sq. km) met their end that day. Disturbingly, if NASA’s frequency estimates are correct, we’re about 3 years overdue for another Tunguska-type strike. Worse, we’re about 64.5 million years past the time when we should have expected another dinosaur-scale hit.
For now, the best defense is to keep our eyes and telescopes trained on the sky. Just what we would do if we saw a big rock coming with time to do something about it is not clear. We’ve probably got the know-how to intercept it en route and blow it up, but that might only yield multiple rocks that would still be big enough to clobber us. We might also try attaching a small rocket to it and nudging it onto a different path. All of that is technologically doable—but we’d best actually get it done. Nothing’s going to change the fact that planet Earth is also target Earth—and always will be.