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Science File

Asteroid Offers a Plot Made for Hollywood

Discovered in 1950 and lost for half a century, the huge rock has a 1-in-300 chance of hitting Earth with the force of millions of tons of TNT, scientists say. But don't wait up for it.


The greatest known threat to Earth from outer space is an asteroid barreling through the universe named 1950 DA. It is six times the size of Morro Rock and capable of ending life as we know it.

We are safe for the time being. So are our great-grandchildren. And theirs. The space rock has a roughly 1-in-300 chance of smacking the planet with the force of millions of tons of TNT and kicking up enough dust to choke off life with a nuclear winter--on March 16, 2880.

"This is not something to worry about," said Jon Giorgini, a Jet Propulsion Laboratory engineer who led the team that analyzed the asteroid and published its findings in this week's issue of the journal Science. "That's 35 generations away."

Rather than being seen as a potential disaster, the finding comes as something of good news in a field better known for ominous forecasts of planetary disasters. More than 1,000 large asteroids capable of hitting Earth are circling through the solar system. Only 576 have been identified.

"We know this thing could not hit until 2880. We can certify the safety," said Steve Ostro, a JPL astronomer who studied the asteroid using radar telescopes. "This is one object we're not losing sleep over."

1950 DA is about two-thirds of a mile wide. The projected impact date falls on a Saturday. Talk about ruining a weekend.

Its impact would generate enough energy to burn forests, spawn huge tidal waves and turn the skies black. But it is far smaller than the asteroid or comet that helped wipe out the dinosaurs 65 million years ago: That one was about 10 miles across.

Typically, astronomers can predict the track of an asteroid for 100 years--and can't say much about what will happen after that. So many factors tweak an asteroid's path--from sunlight to the gravitational tug of stars--that peering further into the future becomes impossible.

"There are many uncertainties and they grow with time," Ostro said.

The JPL team was able to push forecasting limits to nearly 900 years using a blend of good fortune, detailed radar observations and an awful lot of math.

The good luck came in 1950, when the asteroid was observed for 17 days through an optical telescope. To optical telescopes that collect light, dark asteroids are barely perceptible specks. But enough information can be gained from these rough observations to pinpoint their location relative to nearby stars.

After that sighting, the asteroid was lost to science. It was not sighted again until Dec. 31, 2000, when an astronomer at Arizona's Lowell Observatory--who was working instead of ringing in the New Year--saw the asteroid and deemed it a new discovery. Further analysis showed the asteroid was good old 1950 DA, which had eluded telescopes as it looped toward the solar system's outer reaches.

The sightings so many years apart meant astronomers at JPL could discern the current orbit of the asteroid. But the team needed to know more about the rock's position and the rock itself.

The group turned to Ostro and his colleagues, who used the Goldstone Radar Telescope in the Mojave Desert in March 2001 to study the rock.

Radar telescopes like Goldstone glean information by bouncing radio waves off an object. Because of this, they gather information about dim objects that might escape optical telescopes, which rely on light.

The Goldstone radar dish gathered more specific information about the asteroid's orbit and captured an image of the roughly spherical object even though it was more than 4.8 million miles from Earth. The orbit is expected to bring the rock close to Earth a dozen times, but not uncomfortably close until 2880.

The physical attributes of an asteroid determine its traveling plans. The path can be affected by the rock's mass, shape and the way it spins through space. The course of a rock is even altered by its color, which influences the amount of heat it absorbs or reflects from the sun.

Photons from the sun exert a gentle push on the asteroid, Giorgini said. Heat re-radiating away from an asteroid exerts a push in the opposite direction, like a weak rocket. These forces, known as the Yarkovsky effect, are nearly imperceptible. But over time they can nudge the trajectory of a giant rock by millions of miles.

To refine predictions, Giorgini added a multitude of factors--from the shrinking mass of the sun over time to the gravitational tug of Earth, the other planets and even large asteroids--to his equations. The result involved "trillions of calculations," he said.

"It's like predicting a 15-bank shot in a pool game," he said. "It draws on just about every aspect of astronomy, physics and computer science that we know."

The scientists plan to use the calculations on many more near-Earth asteroids they discover and say predicting the fate of space rocks for centuries could become more routine.

The extra time could be put to good use if it becomes necessary to protect Earth.

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