Surprise Asteroid Buzzed Earth Monday

A sequence of four images taken from Australia on February 27, 2009 shows asteroid 2009 DD45, at center, moving across the sky over a 36-minute period.The asteroid came closest to Earth on March 2, 2009, at about 41,000 miles (66,000 kilometers) above the planet's surface.


Sky-watchers in Asia, Australia, and the Pacific islands welcomed a surprise guest Monday: an asteroid that passed just 41,010 miles (66,000 kilometers) above Earth.

Discovered only days ago, asteroid 2009 DD45 zipped between our planet and the moon at 13:44 universal time (8:44 a.m. ET). The asteroid was moving at about 12 miles (20 kilometers) a second when it was closest to Earth.

"We get objects passing fairly close, or closer than this, every few months," Timothy Spahr, director of the International Astronomical Union's Minor Planet Center in Massachusetts, said in an email.

"Also, though, note these are only the ones that are discovered. Many more pass this close undetected"—as asteroid 2009 DD45 nearly did.

Astronomers didn't notice the oncoming asteroid until February 28, when it showed up as a faint dot in pictures taken at the Siding Spring Observatory in Australia.

At that point the asteroid was already a mere 1.5 million miles (2.4 million kilometers) from Earth, and closing in fast.

(Related: "Undetectable Asteroids Could Destroy Cities, Experts Say.")

Asteroids are rocks that generally range from a few feet to several miles in diameter. In our solar system most asteroids orbit the sun in the asteroid belt between Mars and Jupiter.

Asteroid to Buzz Earth Again?

At just 65 to 164 feet (20 to 50 meters) wide, the asteroid "was much fainter than anything visible to the naked eye" even during close approach, Spahr said.

But on Monday observers using backyard telescopes were able to track the asteroid speeding through the constellation Virgo for at least a few hours after the object's closest approach.

According to Spahr, amateur astronomers contributed to the center's monitoring efforts by sending in measurements, which are helping to refine calculations of the asteroid's orbit. Thanks to data from Siding Spring, other observatories, and amateurs, the orbit for 2009 DD24 is "very well determined now," he said.

Astronomers now know that the asteroid is moving within the inner solar system and that the space rock completes an orbit around the sun every 1.56 years.

This means the asteroid could swing close by Earth again someday—though that doesn't seem to be any cause for alarm, if Monday's flyby is any indication.

"As far as we know," Spahr said, "nothing unusual happened.