National Geographic News
The asteroid 2011 MD.

The asteroid 2011 MD streaks past a bright star in a picture taken from the GRAS Observatory in New Mexico.

Image courtesy Ernesto Guido, Nick Howes, and Giovanni Sostero

Dave Mosher

for National Geographic News

Published June 27, 2011

An asteroid the size of a school bus gave Earth an extremely close shave around 1:14 p.m. ET today.

The rogue object—dubbed asteroid 2011 MD—buzzed by at a distance of 7,500 miles (12,000 kilometers) from our planet's surface, or roughly 30 times closer than the moon.

Researchers with MIT's Lincoln Near Earth Asteroid Research (LINEAR) program discovered the asteroid on June 22 and pegged its size between 20 feet (6.3 meters) and 46 feet (14 meters) wide.

Astronomers clocked its top speed at around 63,000 miles (101,000 kilometers) an hour.

Although small by asteroid standards, 2011 MD was close enough for amateur astronomers to spot it with modest telescopes.

Watch video of the asteroid taken June 26 with a 20-inch backyard telescope.

If the asteroid had been on a collision course with Earth, the space rock would have been large enough and fast enough that it would have made it to the ground, said MIT planetary scientist Ben Weiss.

"You'd end up with some sort of explosion and a decent-size crater," he said. "You wouldn't have wanted something like this to land in Manhattan."

Future Asteroid Impact Inevitable

About one asteroid around the same size as 2011 MD comes as close to Earth every five to ten years, and one strikes Earth roughly once every 50 years.

(Related: "NASA to Visit Asteroid Predicted to Hit Earth?")

"This was not an extraordinary event in the world of close asteroid approaches," Weiss said.

"We've recently tracked five other objects that came closer, and a small one of these actually fell to Earth."

That was asteroid 2008 TC3, which was about half the size of asteroid 2011 MD.

Astronomers spotted that object in October 2008, about 24 hours before its fiery entry through the atmosphere. Teams were later able to pick up its pieces in the Sudan desert.

"Sooner or later," Weiss said, "a bigger one is going to hit us."

Newfound Asteroid May Be Back

Asteroid 2011 MD is considered an Apollo-type asteroid, because its orbit is very similar to Earth's yet longer in duration and more oval-shaped.

Astronomers expect the space rock to swing by again in the future—perhaps more closely the next time around—but it's tough to know for certain until the asteroid's departure path is measured in detail.

Short of hitting Earth, the closest asteroid flyby ever recorded was made by a space rock called 2011 CQ1. It passed 3,400 miles (5,480 kilometers) above Earth on February 4, 2011.

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