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Washington DC (SPX) FEB 1, 2006 Two tiny planetoids trailing Jupiter in its orbit could provide new clues about the evolution of the solar system. The binaries � one called Patroclus and the other an unnamed companion � once were thought to be tiny asteroids, but the latest research shows they are made of water ice and a thin layer of dust. Their size and composition closely resemble the comet-like bodies in the Kuiper belt, which lies beyond the orbit of Neptune. "These bodies were supposed to be asteroids and it was thought they would be made of the same material as (Jupiter's moons) Callisto and Ganymede," astronomer and chief researcher Franck Marchis told SpaceDaily.com. Marchis, at the University of California, Berkeley, and colleagues used the super-sensitive optics of the Keck telescope in Hawaii to observe the Patroclus binary. They measured the pair's sizes, distance from each other and orbital speeds around their gravitational center, from which they calculated their respective mass and density. If asteroids, he said, "they'd have a density of 1.9, or a mixture of ice and rock, but the measurements we have show the density is much lower. It is less than 1, so it is similar to the density of comets." Reporting in the February 2 issue of the journal Nature, the team writes that the Patroclus pair supports a particular hypothesis about the solar system's early history that Jupiter and the other gas-giant planets once orbited more closely to the Sun. Back then � before about 650 million years ago � gravitational interaction with one another, and with the giant swarms of asteroids that still surrounded them, caused the Jovian planets to migrate outward, in the process disturbing the orbits of thousands of Kuiper belt objects. Many bombarded the still-forming inner planets, and some were captured at the so-called Trojan points in Jupiter's orbit � where the planet's gravity and the Sun's gravity equalize, and tend to trap passing objects, like air currents create dust bunnies in the corner of a room. Astronomer David Jewitt, of the University of Hawaii in Honolulu, disagreed, however. He told SpaceDaily.com that although the finding of the Marchis team looks solid � "There's not much room for it to be wrong," he said � it doesn't necessarily support a particular solar system evolutionary hypothesis. Jewitt said researchers have long suspected the Trojan asteroids were icy instead of rocky. Orbiting at Jupiter's distance � about five times farther out than Earth � they could have formed in place and did not have to have "fallen in from the Kuiper belt to contain ice," he said. "Density is not destiny, so to speak. The point is that the density is a very blunt instrument when trying to decide where and how an object formed." Marchis said his team will use the Keck instrument to attempt to classify a few more of the Jupiter Trojan objects � of which there are thought to be thousands �beginning this summer. Related Links -
![]() ![]() NASA's Stardust spacecraft was placed into hibernation mode yesterday. Stardust successfully returned to Earth samples of a comet via its sample return capsule on Jan. 15. The spacecraft has logged almost seven years of flight. |
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