On June 13 an asteroid called 2007 XB10 with a diameter of 1.1 kilometers—and the potential to cause major global damage—will zip past Earth. As far as near-Earth objects go, it will pass, fortunately, pretty far, at 10.6 million kilometers, or 27.6 times the Earth-moon distance. Indeed, no giant asteroids appear poised to rewrite history any time soon. The bad news is that we can expect in the next 200 years a small space rock to burst in the atmosphere with enough force to devastate a small city.
A near-Earth object (NEO) is an asteroid or comet that comes within 195 million kilometers of the planet. In 2009 NASA tallied 90 as approaching within five lunar distances and 21 within one lunar distance or less. NEO hunters typically detect them as specks on images, and such momentary glimpses can make their orbits hard to calculate. So researchers can only lay odds of an impact as they await more data. NASA has spotted 940 NEOs one kilometer or more in diameter (about 85 percent of the estimated total of that size), and none will collide with Earth. (The NEO that wiped out the dinosaurs was about 10 kilometers wide.)
The bigger threat now, however, involves the smaller rocks, according to a National Research Council (NRC) report released earlier this year. These asteroids and comets—100,000 or so of them span 140 meters or more—are too small to bring about an Armageddon, but even those at the lowest end of that range could deliver an impact energy of 300 megatons of TNT. And these events occur on average far more frequently (every 30,000 years or so for a 140-meter object) than, say, a one-kilometer impact (every 700,000 years).
On supporting science journalism
If you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.
Given the possible danger, Congress mandated in 2005 that NASA find 90 percent of such NEOs by 2020. But budget shortfalls will make it impossible for scientists to meet that deadline, the NRC has found. NEO hunters get about $4 million in federal funding annually.
In any case, in terms of risk, researchers are thinking even smaller, because the most likely NEO scenario is a 30- to 50-meter-diameter “city killer,” a bolide that would detonate in the atmosphere. The most famous of such devastating “airbursts” occurred in 1908 over Tunguska, Siberia, an event that flattened an area the size of London. The famous Meteor Crater in Barringer, Ariz., resulted from a meteorite in this size category.
At this point, some of the best information on airbursts is kept by the U.S. Department of Defense, Department of Energy and Comprehensive Test Ban Treaty monitoring stations. The NRC report, which asks for more sharing of these closely held data, estimates that 25-meter airbursts occur every 200 years. Most explode over the oceans where the direct risk to life is low but where the initiation of a tsunami is possible. Panel member Mark Boslough of Sandia National Laboratories says a four-meter object blazes in once every year.
And what would we do if we spot a NEO with our name on it? Realistic mitigation plans are in their infancy, says NRC panelist Michael F. A’Hearn of the University of Maryland. For moderately big objects and with notice of years or decades, kinetic impactors make the most sense. The idea is to slam one or more large spacecraft into an object to alter its path. Nuclear detonation is the only option for NEOs exceeding 500 meters across when warning time is months to years.
For city-destroying sizes and short lead times, the choices are limited, perhaps restricted only to evacuation, which we would be lucky to pull off effectively at this point, A’Hearn thinks. All the more reason, it seems, to be thankful that nothing’s headed our way—as far as we know.