Space Menace Looms Centuries Away

A mountain-sized chunk of space rock is the only known large asteroid threatening to hit the Earth, and experts say the potential impact is more than eight centuries away with long odds that it will ever happen.

A new look at an asteroid named 1950 DA shows that it is the most likely of all of the known asteroids at least six-tenths of a mile wide to smash into the Earth, researchers said Thursday in the journal Science.

The study predicts the path of the asteroid as it orbits the sun over many centuries and finds there is a 1 in 300 chance it will hit Earth in the year 2880.

"One in 300 is pretty long odds," said Jon D. Giorgini, a scientist at the Jet Propulsion Laboratory in Pasadena, Calif., and the first author of the study. "I'm not personally going to worry about it. It is so far in the future that lots of things could change."

Tom Morgan, chief scientist of the National Aeronautics and Space Administration's small planet program, said there are an estimated 1,000 asteroids bigger than six-tenths of mile that can pass near the Earth in their orbit of the sun. About 580 have been found and their orbits plotted. Of these, only 1950 DA represents a possible threat. Morgan said NASA continues an effort to identify all the other large asteroids that pass near Earth.

"It is my great hope that we don't find any that are greater threats," Morgan said.

If 1950 DA did hit the Earth, said Giorgini, it would create an explosive release of energy equal to 100,000 megatons of TNT. This would have planet-wide effects, he said, setting off fires, changing the weather and perhaps creating immense tidal waves. But it would not be a planet killer like the asteroid thought to have snuffed out the dinosaurs some 65 million years ago. That asteroid was about 16 times larger than 1950 DA, he said.

In any case, said Giorgini, if scientists determine in the coming centuries that 1950 DA does represent a threat, there'll be plenty of time to take action.

"This is not an urgent thing," Giorgini said. "We can spend a century thinking about it, another century deciding who is going to do something and then another century figuring out what to do. Three hundred years from now - we can't even imagine how they will handle the problem."

Asteroid 1950 DA was first discovered on Feb. 23, 1950, but then not noted in astronomy logs again for decades. It was rediscovered in 2000 and in March 2001 whizzed within about 77 million miles of Earth, giving astronomers a chance to gather visual and radar readings.

From that, the astronomers projected the orbital path 1950 DA would take on its next 15 near passes of the Earth - over a period covering nearly nine centuries.

For the 15th near pass, on March 16, 2880, the analysis showed it was mathematically possible, though unlikely, that the asteroid could hit the Earth.

"What we are predicting is like figuring out a 15-bank shot in a game of pool," said Giorgini. "We can predict the first 13 banks really well, but it is the last few that we need to know more about."

More observations and perhaps close-up views will improve the accuracy of the prediction.

"Once we know more about the physical properties of the asteroid - what it's made of and how it spins - then we can refine that 15th bounce. But it may take decades to get that kind of information," Giorgini said.

He said the highest probability is that the asteroid in the year 2880 will miss the Earth by about 180,000 miles - a distance closer than the 230,000-mile orbit of the Moon around the Earth.

But the range of mathematical probabilities also include a possible impact.

The asteroid's orbit carries it around the sun every 2.2 years. It passes within 77 million miles of the sun and then loops back into space, passing Mars' orbit and reaching a point some 241 million miles from the sun.