A mountain-sized asteroid will make its closest approach to Earth at 9:35 a.m. EDT Sept. 29.

Although asteroid 4179 Toutatis will come no closer than four times the distance between Earth and the Moon (approximately 1.5 million kilometers or 961,000 miles), this will be the closest approach of any known asteroid of comparable size this century.

"This is the closest Toutatis will come for another 500 years, and its orbit is very well known," said Dr. Don Yeomans of NASA's Jet Propulsion Laboratory (JPL), Pasadena, Calif., manager of NASA's Near Earth Objects Program Office. "What this fly-by provides is an opportunity to study one of our closest solar system neighbors," he said.

"While we have done radar observations on this particular asteroid before, this is the closest it has come since at least the twelfth century " said Dr. Steve Ostro, a scientist at JPL. "We will use the huge dish in Arecibo, Puerto Rico, to refine our knowledge of its physical characteristics and its trajectory," he said.

Named after an obscure Celtic and Gallic god, Toutatis, the yam-shaped space rock measures 1.92 kilometers (1.2 miles) by 2.29 kilometers (1.4 miles) by 4.6 kilometers (2.9 miles). Toutatis has one of the strangest rotation states observed in the solar system. Instead of spinning around a single axis, as do the planets and the vast majority of asteroids, it "tumbles" somewhat like a football after a botched pass. Its rotation is the result of two different types of motion with periods of 5.4 and 7.3 Earth days that combine in such a way that Toutatis's orientation, with respect to the solar system, never repeats.

When the asteroid flies past Earth, it will be traveling at approximately 39,600 kilometers per hour (24,550 mph). Toutatis has not passed this close to Earth since the twelfth century, and it will not be this close again until 2562. Toutatis was discovered in 1989.

Arecibo Observatory is operated by Cornell University, Ithaca, N.Y., under a cooperative agreement with the National Science Foundation, with support from NASA.