This is the Web Edition of "A Trip Into Space", a Coimbra-based electronic book on space science. Both the texts and the photos are by courtesy of National Aeronautics and Space Administration.

Asteroid Toutatis
One Of The Strangest Objects In The 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 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 are radar images of asteroid 4179 Toutatis made during the object's recent close approach to Earth. The images reveal two irregularly shaped, cratered objects about 4 and 2.5 kilometers in average diameter which are probably in contact with each other. The four frames shown here were obtained on Dec. 8, 9, 10 and 13, 1992 when Toutatis was an average of about 4 million kilometers from Earth. The time required to obtain each of these images was 55, 14, 37 and 85 min. respectively. On each day, the asteroid was in a different orientation with respect to Earth. In these images, the radar illumination comes from the top of the page, so parts of each component facing toward the bottom are not seen. The large crater shown in the Dec. 9 image (second from the top) is about 700 meters in diameter. The radar observations were carried out at the Goldstone Deep Space Communications Complex in California's Mojave desert by a team led by Dr. Steven Ostro of JPL. for most of the work, a 400,000 watt coded radio transmission was beamed at Toutatis from the Goldstone main 70-meter antenna. The echoes, which took as little as 24 seconds to travel to Toutatis and back, were received by the new 34-meter antenna and relayed back to the 70-meter station where they were decoded and processed into images.


Last Update: 2005-Nov-29