An asteroid is heading for the Earth, and you might want to mark your calendar for 2088 as a good time to be somewhere else, because there is at least a statistical chance of a collision, scientists say.

Of all the asteroids detected to date it is the one most likely to impact our planet, they say -- although they put the odds at one in 300 as a first estimate.

Recent observations have upped the odds to one in 4,000, which sounds better until you realize you have only one in 960,000 chance of being hit by lighting in any given year.

That's why astronomers have made a close study of the asteroid known as 1950 AD, with the surprising finding that it's not a solid body at all but a slack collection of rubble loosely held together by forces so weak they couldn't lift a penny, astronomers say.

It rotates once every 2.1 hours, fast enough that it should be flinging rock off its surface and into space.

That's not happening, so what's keeping the mile-wide asteroid from disintegrating on its own?

It's probably van der Waals forces, weak short-range energies caused by the mutual attraction of molecules with slightly different electrical charges.

If so, that would explain how 1950 DA, a porous rubble pile that's almost half empty space, manages to hold itself together despite its rapid spin.

The findings, published in the journal Nature, could come in handy if we need to deal with such as asteroid that might be an impact risk to Earth, researchers say.

"With such an asteroid, you want to avoid interacting with it directly to prevent it breaking up," University of Tennessee astronomer Ben Rozitis says.

That could simply create an entire swarm of smaller space rocks heading for an impact, the researchers suggest.

If 1950 DA were in fact to hit the Earth, it would do so at a speed of 38,000 mph with an explosive force equal to 44,800 megatons of TNT.

Rozitis and his colleagues calculated the mass of the asteroid at 2.1 trillion kilograms.

Any asteroid spinning as fast as 1950 DA must be fragile, suggesting that if NASA moves ahead with ideas of sending future human exploration missions to asteroids, specimens like 1950 DA might not be the best candidates for such an effort.

No astronaut would feel comfortable standing on such a body, they said.

"With such tenuous cohesive forces holding one of these asteroids together, a very small impulse may result in a complete disruption," says Rozitis.

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