Page last updated at 15:07 GMT, Tuesday, 3 March 2009

Space rock makes close approach

Asteroid Itokawa (Jaxa)
There are much bigger asteroids out there, like Itokawa (pictured)

An asteroid which may be as big as a 10-storey building has passed close by the Earth, astronomers say.

The object, known as 2009 DD45, thought to be 21-47m (68-152ft) across, raced by our planet at 1344 GMT on Monday.

The gap was just 72,000km (44,750 miles); a fifth of the distance between our planet and the Moon.

The rock is of a similar size to the object that many scientists say exploded over Siberia in 1908 with the force of 1,000 atomic bombs.

The object was first reported on Saturday by the Siding Spring Survey, a near-Earth object search programme in Australia.

It was confirmed by the International Astronomical Union's Minor Planet Centre (MPC), which catalogues Solar System objects.

The closest recent flyby listed by the MPC is 2004 FU162, a small asteroid about 6m (20ft) across which came within about 6,500km (4,000 miles) of our planet in March 2004.

The latest object, 2009 DD45, passed by our planet at only twice the altitude of satellites in geosynchronous orbit.

Surprise miss

In 1908, an object with a similar size exploded over Siberia, flattening 80 million trees over an area of 2,000 square km (800 square miles) near the Tunguska river.

"There is still a lot of debate over how big the Tunguska object was," Professor Alan Fitzsimmons, an astronomer at Queens University Belfast, told BBC News.

"It was always thought to be 50 or 70m (165-230ft) across. But some recent calculations have implied it may have been even smaller than that - maybe down to 30m (100ft) in size. There's a large uncertainty there, but it puts (this object) in the same ballpark."

Felled trees at Tunguska (University of Bologna)
Felled trees can still be seen today at the Tunguska site

A United Nations working group on near-Earth objects (NEOs) met last month to discuss drafting international procedures for handling the asteroid threat.

Dr Richard Crowther, chair of the UN Working Group on Near-Earth Objects, commented: "Although we will meet formally again in June of this year to advance our work on this important issue, I am sure we will discuss the implications of the 2009 DD45 close approach informally before then.

The expert, who works for the Science & Technology Facilities Council (STFC), told BBC News: "Such unanticipated near-misses - at least in astronomical terms - from objects such as 2009 DD45... demonstrate the need for the global community to establish the means to mitigate this impact threat."

As had been shown with the Tunguska object, asteroids of this size could potentially unleash a destructive power equivalent to about 10 to 15 megatonnes of TNT, Dr Crowther explained. This is about 1,000 times more powerful than the blast from the bomb dropped on Hiroshima.

Professor Fitzsimmons added that the destructive potential of an object such as 2009 DD45 depended on what it was made of and the angle at which it hit the Earth's atmosphere.

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