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Science & Space

City-sized asteroid to clear Earth

By Robert Roy Britt
SPACE.comexternal link

Computer model of the asteroid Toutatis based on radar data.
Computer model of the asteroid Toutatis based on radar data.
National Aeronautics and Space Administration (NASA)
Space Exploration

(SPACE.comexternal link) -- A minor rumor has hatched on the Internet that a large and deadly asteroid will strike Earth this fall. Bulletin board discussions cite a 63 percent chance of impact.

Astronomers know of no such impending doom.

The rumors are likely rooted in a real event, however. On Sept. 29, 2004 an asteroid the size of a small city will make the closest known pass of such a very large space rock anytime this century.

While not dangerous for now, asteroid Toutatis is incredibly strange. And scientists are quite familiar with it, having bounced radar off the tumbling stone on previous flybys to generate computer renderings of its weird shape and movement.

Toutatis looks something like a dumbbell hurtling awkwardly through space. It has a crazy rotation that makes normal days impossible. Scientists can't explain the shape or the spin, but they're eager to learn more in September when, during the close pass, even backyard skywatchers will be able to spot the asteroid.

Well known path

The orbit of Toutatis is pinned down with better precision than any other large asteroid known to cross Earth's orbit. Toutatis' 4-year trek around the sun ranges from just inside the Earth's path out to the main asteroid belt between Mars and Jupiter. The asteroid visits us every four years.

This fall, it will zoom by our planet within a million miles, or about four times the distance to the moon.

That's close by cosmic standards for an object that could cause global devastation. Toutatis hasn't been so near since the year 1353 and won't be that close again until 2562, NASA scientists have calculated. No other asteroid so large is known to have come so close in the past, though accurate tracking of space rocks is a fairly recent, high-tech skill that still leaves wide margins of error for many objects.

Toutatis is about 2.9 miles long and 1.5 miles wide.

Many smaller space rocks have passed by much closer, well inside the Moon's orbit. Other asteroids in the size range of Toutatis have surely navigated that window, too, but were unseen in eras when the skies were not scanned so fully as today.

And throughout history, several asteroids and comets have hit the planet. In fact, an object the size of Mars hit Earth when it was very young, creating the Moon, scientists believe. But experts say the odds of a major collision in any year are extremely small. Any other near-Earth asteroid as big as Toutatis would almost surely be spotted decades or centuries before any possible impact.

The prediction of any such event would make huge news rather than small rumors.

Not dangerous, just bizarre

Asteroid Toutatis, officially numbered 4179, was discovered by French astronomers in 1989. Researchers can't predict far enough into the future to rule out Toutatis ever slamming into Earth, so it is listed officially as a Potentially Hazardous Asteroid. NASA says it won't hit for at least the next six centuries.

Meanwhile, previous close approaches have allowed intriguing radar examinations of one of the oddest things in space.

"The vast majority of asteroids and all the planets spin about a single axis, like a football thrown in a perfect spiral," explains Scott Hudson of Washington State University. "But Toutatis tumbles like a flubbed pass."

The result is a lack of anything resembling a normal day or night on the giant, pockmarked space rock.

Instead of a fixed north pole, Toutatis' axis of rotation wanders around in two separate cycles of 5.4 and 7.3 Earth-days. Stars seen from any location on the asteroid "would crisscross the sky, never following the same path twice,'' Hudson says.

More study planned

Steven Ostro at NASA's Jet Propulsion Laboratory has, with Hudson, studied Toutatis via radar on previous flybys. Ostro said that the population of near-Earth asteroids -- hundreds bigger than 0.6 miles (1 kilometer) have been found in the past six years or so -- are now known to come in "a zoo of shapes." And there are other asteroids that don't rotate on a single, main axis.

"But Toutatis remains the only non-principal-axis rotator in the solar system whose shape and spin state are well defined," Ostro said. More radar observations this year will try to further refine the spin rate and orbit.

There is more to learn. For starters, scientists also can't yet say if Toutatis has a hard surface or a thick layer of loose dirt similar to the moon.

"I'd very much like to know whether Toutatis' strange shape and ponderously slow, wobbly rotation are the result of collisional breaking apart or a gentle merger of the asteroid's two lobes, and when the responsible phenomena happened," Ostro said.

Answers to all these big questions might require an as-yet-unplanned visit.

"Because of the radar investigations, our physical characterization of Toutatis is the best we have for any Potentially Hazardous Asteroid," Ostro said. "But a spacecraft rendezvous could tell us a great deal more, and I would love to see this happen."

Looking both ways

On Sept. 29, backyard skywatchers on Earth can find Toutatis, providing they know where to look.

Toutatis won't be visible to the unaided eye. Ordinary binoculars should be sufficient for spotting it if the sky is clear and dark, says Alan Harris, of the Space Science Institute in Boulder, Colorado.

"However, to actually know what your seeing, a small telescope would be useful," Harris says. That will allow you to detect the slow motion of Toutatis against background stars. The asteroid will appear as a point of light, much like a star. It is too far for surface details to be visible.

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