A freshly discovered asteroid, roughly as long as a tennis court, will zoom past Earth at about the distance of the moon Thursday, according to NASA. The space rock, called 2010 GA6, was first observed Monday by the Catalina Sky Survey, a telescope project in Arizona that seeks out near-Earth asteroids and comets. 2010 GA6 will make its closest approach to Earth, at a distance about 430,000 kilometers, at 10:06 P.M. Eastern Daylight Time.

The proximity of 2010 GA6's approach is not unique; in 2010 three other asteroids have come as close or closer to Earth. But the newfound visitor is the brightest asteroid, and consequently among the largest, to draw so near in the past year. Its brightness indicates an approximate diameter of 20 to 40 meters; the next brightest to pass at or within lunar distance in that time span was 2009 JL2, an asteroid about 17 to 37 meters across, in May 2009.

It is not unusual that 2010 GA6 was discovered so soon before reaching Earth's vicinity; asteroids that small are difficult to spot at great distances. No other approaches of lunar distance or closer are known to be imminent in the next year, despite the fact that they occur every month or so. In other words, plenty of asteroids are headed this way—they simply have not been spotted yet, and asteroid watchers are more focused on the larger objects that pose greater threats to life.

In 1998 Congress charged NASA with finding kilometer-size and larger asteroids that draw close to Earth. A good portion of that population has now been catalogued, and the scope of the survey has since expanded to include objects down to 140 meters in diameter, only a fraction of which have been found. Even smaller objects of the 2010 GA6 variety probably number in the millions, and they could still do significant local damage with an impact. There is a roughly 50 percent chance of a 30-meter-plus asteroid striking Earth each century, according to Clark Chapman, a space scientist at the Southwest Research Institute in Boulder, Colo. Chapman told ScientificAmerican.com in 2009 that such an asteroid impact would cause a multimegaton atmospheric explosion over Earth's surface, rather than impacting it. "It could be quite damaging (and even lethal) out to distances of 10 to 20 kilometers in all directions if it happened over a populated region with weak structures," Chapman said.

Orbit diagram of 2010 GA6 (blue dot) and Earth (green dot): NASA