Planet Earth experienced a (relatively) close call today when an asteroid named “2011 MD” passed us by, missing by roughly 7,500 miles. 2011 MD is not an especially dangerous asteroid—at roughly 30 feet wide, it would be too small to do any serious damage in the case of an impact. But Earth is no stranger to encounters with so-called Near-Earth Asteroids (NEAs), which are asteroids “that travel to within 1.3AU” (about 121 million miles) of the sun. Fortunately, these strikes have become far less common; in Earth’s early days, scientists estimate that the impact rate was thousands of times higher, probably resulting in strikes even larger than the “Chicxulub event” that killed the dinosaurs about 65 million years ago. What are the odds that such an event could occur today?
Thankfully, they’re not very high. The most destructive NEAs, labeled “civilization destroyers” by Clark Chapman of the Southwest Research Institute, are 2-3 kilometers in diameter. But Chapman points out in a 2004 paper that because these NEAs are so large, scientists have already located most of them—and in most cases we would likely have years of warning to prepare for, or attempt to deflect, an impact. Chapman puts the odds of such a strike occurring in the next century at less than 1 in 100,000. That said, he argues that Earth exists in a “cosmic shooting gallery,” and while impacts may be extremely rare, they can also be extremely disastrous. In the meantime, though, there’s little reason to worry: NASA reports that “the only known asteroid whose hazard could be above the background level” is Asteroid 1950 DA, which has a collision probability that ranges from 0 to 0.33 percent. And in any case, it’s not scheduled to pass near earth until March 16, 2880.