An asteroid, nearly a mile wide in diameter, could strike Earth in 2880, scientists say. (NASA)

The bad news is an asteroid nearly a mile wide could be on a collision course with Earth. The good news is we have time - plenty of time.

Scientists writing in the journal Science say there is a one in 300 chance that the asteroid 1950 DA will collide with Earth on March 16, 2880. An encounter would be the equivalent of smashing a million tons of TNT into Earth and could wipe out a large city, trigger widespread fires and tidal waves.

Why worry about an event that might happen some 35 generations into the future? Lead author Jon Giorgini of NASA's Jet Propulsion Laboratory explains he isn't trying to sound a warning bell.

Instead, he wants to get researchers thinking about how to improve predictions of asteroid collisions - and how to prevent them.

"It's so far in the future that it's nothing anyone should worry about now," he said. "But it illustrates the value of understanding things sooner rather than later."

Astronomers Who Cried Wolf

Astronomers have warned of possible asteroid collisions in the past. Perhaps most notorious was the 1998 announcement from a scientist at the International Astronomical Union that a massive asteroid would smash into Earth in 2028. After much public hand-wringing, NASA dismissed the prospect as impossible a day later.

The uncertainty of Armageddon announcements generally comes from a lack of data. Although NASA has set up a program to identify and monitor all of the large asteroids that pass near Earth, so far astronomers have only located about half of these estimated 1,200 potentially hazardous asteroids. Additional observations often quickly reveal asteroids as non-threatening.

But asteroid 1950 DA is different. Thanks to a quirk in the asteroid's orbit, observation records dating back to 1950 and precise radar readings from NASA's Arecibo station in Puerto Rico when the asteroid hurtled by more than 70 million miles from Earth in 2000 and 2001, researchers understanding of this asteroid and its path of orbit is unusually complete.

"This one is distant in the future but it's most interesting because it's not likely to disappear after a few observations," said Steven Chesley, a coauthor of the Science paper and a researcher at the Jet Propulsion Laboratory.

Astronomers have learned the asteroid swings around the sun every 2.2 years. It passes within 77 million miles of the sun and then streaks back into space, passing Mars' orbit and reaching a point about 241 million miles from the sun. The rocky body is 1.1 km (0.68 miles) in diameter, it has at least a couple large craters on its surface and it spins very fast - about once every 2.1 hours.

"If you were to stand on the surface you might get dizzy watching the stars zip by," said Giorgini.

Fuzzy Factors

That said, there's still a lot that scientists don't know about 1950 DA, including small factors that, over 878 years, could significantly skew predictions.

They don't know, for example, the exact mass of the asteroid or its rotation or its thermal conductivity. Even the computing tools astronomers use and the way programs round off to the nearest number after so many digits muddies the prediction of an encounter.

Giorgini says the biggest wildcard is a factor called Yarkovsky's effect. This effect, believed to be first penned by a Polish engineer named IO Yarkovsky in the early 1900s, describes how unbalanced heating of a small body can propel it in a particular direction.

Like on Earth, the surface of an asteroid experiences seasons, with a "summer" in which the northern hemisphere is hotter. It also experiences "afternoons" (on 1950 DA an "afternoon" would amount to minutes rather than hours) that are warmer than "mornings."

The warmer regions of an asteroid absorb more thermal energy and so give off more thermal energy. The radiation propels the asteroid slightly, like a rocket.

"If you add the Yarkovskey effect, it could be this asteroid doesn't have a chance of hitting us," said Joseph Spitale of the Lunar and Planetary Laboratory at the University of Tucson, Ariz.

So far scientists still aren't sure how or even if the Yarkovsky effect will change the trajectory of 1950 DA. Unknown factors like the roughness of the asteroid's surface (dirt absorbs heat less readily than flat rock) and the asteroid's coloring (a darker surface will absorb more heat) all influence the effect on the asteroid.

It may be the only way to fill in these blanks will be by sending a robotic or manned mission to the asteroid.

"We now know what we need to know," said Giorgini. "That's important in science - you've got to know your limits."

Devising Plan Deflection

Spitale, meanwhile, has been thinking of ways of using the Yarkovsky effect to mankind's advantage. He suggests if we know far in advance that an asteroid is heading our way, we might be able to deflect the rock by altering the asteroid's surface and changing the Yarkovsky effect on the body.

Spitale suggests this could be done by blanketing the surface with a layer of dirt to decrease how readily it absorbs heat. Or, he says, we could "paint" the asteroid white - perhaps by dropping tons of white powder across its surface - to increase its reflectivity. Another alternative is to break up the asteroid's surface using a series of planted explosives.

"All you have to do is change properties like its thermal conductivity or its color and that will affect its trajectory," Spitale said.

Altering the asteroid's surface may sound like an elaborate, expensive task, but it's simple compared to other suggestions. Some have proposed dropping a shrink-wrap material on an asteroid, attaching a giant solar sail to it or blasting it with a nuclear bomb - as characters did in the 1998 movie Armageddon.

No matter the method, Spitale believes it's vital that people develop some method of deflecting big incoming rocks, whether we use it in the next few decades or 878 years in the future.

"It definitely is a credible threat," he said. "We know an asteroid caused mass extinction among the dinosaurs 65 million years ago. These things happen rarely, but they do happen."

(China Daily April 5, 2002)