Today is the hundredth anniversary of the Tunguska event, when a massive explosion (likely resulting from a small asteroid or comet) rocked a remote region of central Siberia, flattening trees and reportedly knocking people to the ground in the nearest settlement 40 miles from the blast. Science News and Scientific American have had articles recently highlighting the latest developments in the ongoing debate about what caused the blast. A team of Italian researchers claimed last year that a lake in the blast zone might be concealing an impact crater, though (as the Science News article reports) that theory has come under attack. Other theories for what caused the explosion--some more plausible than others--include a small black hole, a bit of antimatter, a UFO, or a kimberlite eruption bringing diamonds and methane to Earth's surface.
One sobering bit of information is that if the blast was, in fact, a result of a comet or asteroid detonating in Earth's atmosphere, it was a very small one. In his recent article in the Atlantic, Gregg Easterbrook reported that current evidence indicates that it may have been as tiny as 30 meters in diameter. By contrast, other asteroids scientists are tracking that have an outside shot at colliding with Earth are larger: One that could strike the planet in April 2036 has a diameter of 300 meters, and would destroy an area the size of France. And that's to say nothing of the global consequences. Because the Tunguska body detonated before striking the Earth, it didn't have a chance to kick up much debris into the atmosphere. An impact from a larger asteroid could cause widespread acid rain and crop failures around the globe for several years after impact.
A good reason why--as Easterbrook suggests--NASA might want to devote a bit more time and energy to the threat posed by near-Earth space rocks. Unfortunately, as former astronaut Russell Schweickart told Andrew Revkin of The New York Times, quite a few members of Congress who privately support more funding for asteroid research are afraid to vote in favor of it, lest their opponents use the vote to make them look like paranoid crazies come election time.