On September 29, Toutatis will be within a million miles of Earth, or about four times the distance to the Moon.

No space rock this big will pass so close in the next century, scientists say. And while similarly large asteroids have hit the planet in the distant past, none so big have come so close since astronomers have had the means to notice them. Many smaller space rocks have been spotted much closer, even inside the orbit of the moon.

NASA scientists and other asteroid experts have been watching Toutatis for more than a decade, and though its orbit changes slightly with each 4-year trip around the sun, they have a good handle on the path.

The position of the asteroid on this pass is known to a precision roughly equal to the rock's size, said Alan Harris, a senior research scientist at the Space Science Institute. That leaves a little wiggle room for its exact location at closest approach, but not much.

"Because of the nature of the orbit, we cannot predict thousands of years into the future for this object, but in anyone's lifetime now, there is no chance" of an impact, Harris said.

Spotting Toutatis

Toutatis will not be visible to the unaided eye. Experienced telescope users can see it now from the Southern Hemisphere, and in early October it will be visible from the north.

Finding Toutatis will be challenging, Harris said, due to a combination of the asteroid's position in the sky and interfering moonlight.

Because the asteroid is so close, its location in the sky will vary significantly for skywatchers in different places on Earth at any given moment. And because it moves quickly, the location changes constantly. Printed sky maps don't always provide enough detail to be useful.

"In a large telescope the motion would be perceptible against any stars in the field more or less in real time, sort of like watching the second hand on a clock," Harris said, adding that the movement would be "not quite that fast, but noticeable."

Highly experienced observers will use complex plotting information known as ephemeris data. Others can use software programs that generate maps for specific times and locations.

When, where and how

At its closest on September 29, Toutatis will be visible only to observers in the Southern Hemisphere.

Large and steady binoculars will be able to pick out the pinprick of sunlight reflecting off the asteroid, providing observers "use a good program like Starry Night Pro to plot its incredibly rapid motion across the sky," said Clay Sherrod of the Arkansas Sky Observatory. (The software company Starry Night is owned by Imaginova, parent also of SPACE.com.)

Soon thereafter, experienced backyard astronomers north of the equator will have a chance to find Toutatis.

"By early October, it will suddenly be re-emerging into northern skies as its apparent trajectory will bring it back into very favorable view," Sherrod said in an e-mail interview. But by then the asteroid will be moving away from Earth and getting dimmer. It will quickly become "very difficult" to spot even with an 8-inch telescope, he said.

Sherrod photographed the giant space rock last week (it was visible then in the north through large telescopes) and said exposures longer than eight seconds showed a trail as the giant rock moved slightly against the background of stars.

"It has been quite a wonderful show so far," he said.

Strange rock indeed

Asteroid Toutatis was discovered in 1989. Scientists have modeled its strange rotation and odd shape -- it looks something like a pockmarked dumbbell -- on previous flybys.

Instead of a fixed north pole, Toutatis' axis of rotation wanders in two separate cycles of 5.4 and 7.3 Earth-days. So while most asteroids rotate somewhat like a football thrown in a perfect spiral, "Toutatis tumbles like a flubbed pass," says Scott Hudson of Washington State University.

Astronomers will use this week's flyby to examine Toutatis in greater detail, with a goal of pinning down the rock's rate of spin and better estimating its future path.

While some rumors have suggested the asteroid's forecasted course might be off by enough to cause a collision with Earth, Sherrod agrees with Harris and other scientists that there is no chance for calamity. Sherrod has been monitoring Toutatis' movement since July 3, logging more than 500 observations that allow mapping of a precise trajectory.

"Although the actual path of it has indeed varied a slight bit from the original calculated, there is absolutely no chance of a physical encounter or impact with Earth," he said.

http://edition.cnn.com/2004/TECH/space/09/28/asteriod.fly/index.html