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 is 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 (JPL), 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 asteroids 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.
The scientists' computer model reveals Toutatis to have dimensions of 2.9 miles by 1.5 miles by 1.2 miles. Numerous surface features, including a pair of half-mile- wide craters, side by side, and a series of three prominent ridges -- a type of asteroid mountain range -- are presumed to result from a complex history of impacts.
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 by Earth at a range of four times the distance between the Earth and the Moon, 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, the orbit of Toutatis 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, DC.
These images show a computer model of the Earth-crossing asteroid 4179 Toutatis. The model was created by Drs. Scott Hudson of Washington State University and Steven Ostro of the Jet Propulsion Laboratory using radar data that had been obtained with the Goldstone radar telescope in California and the Arecibo radar telescope in Puerto Rico. The object is about 4.6 kilometers (3 miles) long. The resolution of the computer model is about 84 meters. The views of the asteroid show shallow craters, linear ridges and a deep topographic "neck" whose geologic origin is not known. It may have been sculpted by impacts into a single, coherent body, or Toutatis might actually consist of two separate objects that came together in a gentle collision. Toutatis has one of the strangest rotation states yet observed in the solar system. Instead of the spinning about 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 way that Toutatis' orientation with respect to the solar system never repeats.
Hudson and Ostro's model appeared in the October 6, 1995 issue of Science. Their work was part of the Planetary Geology and Geophysics Profgram and the Planetary Astronomy Program of NASA's Office of Space Science.
DESCRIPTION OF VIDEO
This video shows a computer model of the Earth-orbit- crossing asteroid 4179 Toutatis rotating in space. Each sequence uses illumination from a different direction.
(Please send an email to Scott Hudson for information about how to obtain this video. He will respond to emails after the DPS meeting.)
Toutatis images and brief captions are available on the World Wide Web at
The TIF versions are 24-bit color and will give the best image quality. Please contact Scott Hudson if you experience any difficulty.