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Space rock buzzes past Earth

A NASA graphic traces the asteroid 2009 VA's path within the moon's orbit and past

Earth. Each dot on the 2009 VA line indicates an hour of time along the route.

Asteroid-watchers say a space rock about as big as a garage came within 9,000 miles (14,000 kilometers) of Earth last Friday, just 15 hours after it was detected.

Experts quickly determined that the asteroid 2009 VA would miss us - and even if it came directly at us, it wouldn't have caused a catastrophe. Nevertheless, the close encounter serves as a reminder that someday a much bigger rock may well hit us and that it's best to be prepared.

In this week's recap of the event, NASA's Near-Earth Object Program Office reported that 2009 VA came well within the moon's orbit - so close, in fact, that the asteroid's orbital path was bent by Earth's gravitational pull.

NASA and other space agencies around the world have been keeping increasingly close track of near-Earth asteroids and comets, with a strong assist from amateur astronomers. In this case, the object was first detected by the Catalina Sky Survey at the University of Arizona. It was quickly identified by the Minor Planet Center in Cambridge, Mass., as a close-approaching asteroid. Then NASA experts worked out its orbit and gave the all-clear.

Why wasn't the rock found sooner? Well, smaller objects are more difficult to detect in advance, and this one was estimated to be only 7 meters (23 feet) wide. That's nowhere near as big as the 10-kilometer-wide (6-mile-wide) object that apparently did in the dinosaurs 65 million years ago - or even the 30-meter-wide (100-foot-wide) Tunguska object that was thought to have wreaked destruction in a Siberian forest in 1908.

For what it's worth, the Defense Department's Joint Space Operations Center tracks about 19,000 orbital objects down to the size of 10 centimeters (4 inches), and NASA tracks bits of space junk that are even smaller. But incoming near-Earth objects are trickier to track until they're almost upon us.

In the close-but-no-collision category, this one was No. 3 on NASA's list for cataloged asteroids: A meter-wide (yard-wide) asteroid came within 6,150 kilometers (3,821 miles) in October 2008, while another space rock about the size of 2009 VA passed within 6,535 kilometers (4,060 miles) in March 2004.

If 2009 VA had entered the atmosphere, it almost certainly would have blown itself up before hitting the ground - just as a larger asteroid did a month ago, without warning, in the skies over Indonesia. A somewhat smaller asteroid met a similar fate in the skies over Africa about 13 months ago. (Months later, students in Sudan found 4 kilograms (8.7 pounds) of meteorites that fell to Earth after last year's blast.)

Such atmospheric blow-ups release energy equivalent to about a kiloton of TNT. In comparison, the Hiroshima atomic bomb set off a roughly 15-kiloton blast.

So, for several reasons, we shouldn't hit the alarm button over 2009 VA. But that doesn't mean we should hit the snooze button, either: The Indonesia blast and the surprise pummeling that Jupiter took back in July are just foretastes of nasty surprises that could be waiting for us. The more we know about asteroids and how to fend them off, the better. Here are some reports that lay out the asteroid threat and what NASA has been doing about it:

Update for 3:35 p.m. ET: I've upped the estimate for the dino-killing asteroid to a whopping 10 kilometers (6 miles) across. Anything bigger than 1 kilometer wide would be considered capable of causing a global catastrophe.

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