Montage of our solar system

Two NASA-sponsored scientists studying the Earth-crossing asteroid 4179 Toutatis with radio telescopes have found it to be one of the strangest objects in the solar system, with a highly irregular shape and an extraordinarily complex "tumbling" rotation.

Both its shape and rotation are thought to be the outcome of a history of violent collisions. A detailed description of the asteroid and its observed rotation are reported in this week's issue of the journal Science, by Drs. Scott Hudson of Washington State University and Steven Ostro of NASA's Jet Propulsion Laboratory, Pasadena, CA.

"The vast majority of asteroids, and all the planets, spin about a single axis, like a football thrown in a perfect spiral," Hudson said, "but Toutatis tumbles like a flubbed pass."

One consequence of this strange rotation is that Toutatis does not have a fixed north pole like the Earth. Instead, its north pole wanders along a curve on the asteroid about every 5.4 days. "The stars viewed from Toutatis wouldn't repeatedly follow circular paths, but would crisscross the sky, never following the same path twice," Hudson said.

"The motion of the Sun during a Toutatis year, which is about four Earth years, would be even more complex," he continued. "In fact, Toutatis doesn't have anything you could call a 'day.' 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."

The rotations of hundreds of asteroids have been studied with optical telescopes. The vast majority of them appear to be in simple rotation with a fixed pole and periods typically between one hour and one day, the scientists said, even though the violent collisions these objects are thought to have experienced would mean that every one of them, at some time in the past, should have been tumbling like Toutatis.

Internal friction has caused them to change into simple rotational patterns in relatively brief amounts of time. However, Toutatis rotates so slowly that this "dampening" process would take much longer than the age of the solar system. This means that the rotation of Toutatis is a remarkable, well-preserved relic of the collision-related evolution of an asteroid.

Hudson and Ostro used radar images obtained with the Deep Space Network Goldstone radar antenna in California and the Arecibo telescope in Puerto Rico in 1992, when Toutatis passed to within a little more than 2 million miles of the Earth. The images are reported in a companion paper, also in this week's issue of Science.

Toutatis was discovered by French astronomers in 1989 and was named after a Celtic god that was the protector of the tribe in ancient Gaul. Its eccentric, four-year orbit extends from just inside the Earth's orbit to the main asteroid belt between Mars and Jupiter. The plane of Toutatis's orbit is closer to the plane of the Earth's orbit than any known Earth-orbit-crossing asteroid.

On September 29, 2004, Toutatis will pass four lunar distances from Earth, the closest approach of any known asteroid or comet between now and 2060. One consequence of the asteroid's frequent close approaches to Earth is that its trajectory more than several centuries from now cannot be predicted accurately. In fact, of all the Earth-crossing asteroids, Toutatis's orbit is thought to be one of the most chaotic.

Earth-crossing asteroids are of great interest to scientists for their relationships to meteorites, main-belt asteroids and comets; as targets of human or robotic exploration; as sources of materials with potential commercial value; and as long-term collision hazards. Nearly 300 Earth-crossing asteroids have been discovered, but the entire population is thought to include some 1,500 objects larger than one kilometer and some 135,000 objects that are larger than 100 meters.

The scientists' work was funded by the Planetary Geology and Geophysics Program and the Planetary Astronomy Program of NASA's Office of Space Science, Washington, D.C.

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