Paging Bruce Willis - By J. Dana Stuster

Not to be overly alarmist, but Earth has had two near-misses with large asteroids so far this month. On Wednesday, a hunk of space rock three miles in diameter and going by the designation 4179 Toutatis passed by the planet, missing by 4.3 million miles, or about 18 times the distance from the Earth to the moon.  The day before, an approximately 100 foot-wide asteroid passed much closer -- within 140,000 miles, slipping between the Earth and the moon. Neither of these are the Planet X that some New Age doomsayers claim will bring the end of the world on December 21 (the date the ancient Mayan calendar ends) nor are they quite on the scale of the six-mile wide asteroid that wiped out the dinosaurs -- but still, even a "small" asteroid impact could cause tsunamis, skyline-flattening blasts, and at an extreme, nuclear winter.

The problem is that while the effects might be monstrous, in the cosmic scale of things these "near-Earth objects" (NEOs) are tiny, and thus difficult to detect. XE54, the 100-foot asteroid that passed by on Tuesday, wasn't discovered until two days before its fly-by. The effect of an NEO impact would have global implications, but whose responsibility is it to find these space rocks and, if necessary, to stop them?

At present, NEOs are discovered and tracked by an informal international network of professional and amateur astronomers. Their findings are cataloged and published by the Minor Planet Center, which is run by the Smithsonian and has the support of the International Astronomical Union, the leading professional society of astronomers. In addition, the U.S. government has legally required NEO research. In 2005, Congress mandated that NASA identify at least 90 percent of all NEOs wider than 450 feet by 2020, though, as a 2011 report by the National Academy Sciences points out, Congress did not appropriate funds to meet this goal.

NASA's current efforts include surveys to identify the largest NEOs, and partnerships with some military- and university-operated telescopes. NASA is also cooperating with other countries' projects, including the Japan Aerospace Exploration Agency's Hayabusa space probe, which intercepted asteroid 25143 Itokawa in 2005 and returned samples to Earth, and the Canadian Space Agency's Near-Earth Object Surveillance Satellite, scheduled for launch next year.

NASA/JPL-Caltech

Data from NASA's Wide-field Infrared Explorer (WISE), an orbiting space telescope, has led astronomers to estimate that there are approximately 20,000 NEOs with diameters greater than 330 feet. This is an improvement over previous estimates, but even with the revised lower numbers, astronomers have found maybe only one-fourth of the space rocks in Earth's path. Worse, when they are identified, it's often only a few days before they pass unnervingly close to home. If one of these NEOs were on a collision course, options to deflect it would be limited -- and though international law regulates issues of sovereignty and military use in space, there is no international framework for whose responsibility it would be to avert an asteroid impact. If the United States has a plan, it's not public.