Asteroid 2002 MN, estimated at up to 120 metres (yards) long, hurtled by the Earth at a distance of 120,000 kilometers (75,000 miles), well within the orbit of the Moon and just a hair's breadth in galactic terms.

It is the closest recorded near-miss by any asteroid, with the exception of a 10-metre (33-feet) rock, 1994 XM1, which approached within 105,000 kilometers (65,000) miles on December 9, 1994, they said.

"2002 MN is a lightweight among asteroids and incapable of causing damage on a global scale, such as the object associated with the extinction of the dinosaurs," the Near Earth Object (NEO) Information Centre of Britain's National Space Centre said in a press release.

"However, if it had hit the Earth, 2002 MN may have caused local devastation similar to that which occurred in Tunguska, Siberia in 1908, when 2,000 square kilometres (800 square miles) of forest were flattened," it said.

Spokesman Kevin Yates told AFP that the asteroid was only spotted on June 17 -- three days after its flyby.

Had it collided with the Earth, "the most likely thing is that it would have detonated in the atmosphere, creating a blast wave," he said.

"You're talking in the region of 10 megatonnes -- quite a lot of energy to be released in any one place," he said.

The risk of the Earth being hit by an asteroid or comet is very remote, and most objects never come so close as 2002 MN.

NASA's Near-Earth Object (NEO) Program website confirmed the incident and said 2002 MN was spotted by the Lincoln Near Earth Asteroid Research (LINEAR), a project funded by the US Air Force and NASA and located in New Mexico.

The website of the US magazine Sky et Telescope voiced alarm at the near miss.

"What is most shocking is just how close it came to Earth," it said.

"Though the exact details of an impact scenario depend on the rock's composition, had it hit the Earth, the event would have been 'Tunguska-like', with a force rivalling the largest H-bombs."

US and other astronomers are working hard to map large asteroids, greater than a kilometer (five-eighths of a mile) across, that could inflict lasting climate change.

One such monster is believed to have wacked into the Earth 65 million years ago in what is modern-day Mexico, kicking up dust and debris that swathed the planet, unleashing a prolonged winter that ended the long reign of the dinosaurs.

But many specialists are worried that little sustained effort is being made to spot smaller space wanderers, which could still unleash the energy of an arsenal of nuclear bombs if they collided with our home.

In addition, the search for dangerously asteroids is overwhelmingly conducted by telescopes in the northern hemisphere. A rock approaching from the southern hemisphere could go undetected.

Astronomers spot asteroids thanks to the light they reflect from the Sun, which means that smaller ones are frequently only discovered when they are very close to the Earth and become visible.

If one of these were on a collision course, that would leave no time to launch a rocket or missiles to try to deflect or destroy it, or even prepare cities for a potential disaster.

Asteroids are often described as the rubble left over from the building of the Solar System.

They orbit the Sun, but the paths are never eternal, for the trajectories can be deflected by gravitational pull whenever the asteroid passes by a planet or goes around the star itself.

The latest calculations of 2002 MN suggest it has an orbit of 894.9 days and is unlikely ever to be any future threat to the Earth, said Yates.

The next close flyby will be in 2061 but the distance will be much greater than in the June 14 episode, he said.

Asteroids are a very remote yet real peril, because they move at such speeds that they unleash terrific energy on impact.

The Tunguska event was caused by an object estimated to be 60 metresfeet) long. It exploded in the atmosphere with the force of 600 times the Hiroshima bomb.

SPACE.WIRE