What's the source of quick-return comets?

New observations from the Hubble Space Telescope are demonstrating that scientists don't know where a major class of comets comes from.

Until recently, most planetary scientists had assumed that comets that take less than 20 years to orbit the sun originally resided in the Kuiper belt, a doughnut of icy material left over from the formation of the planets 4.5 billion years ago. The belt lies just beyond the orbits of Neptune and Pluto. But new observations suggest that to serve as a storehouse for comets, the Kuiper belt is too thinly populated with suitable objects.

Last winter, Hubble's Advanced Camera for Surveys stared for 200 hours at a tiny region of sky, just 10 percent the size of the full moon as seen from Earth. Gary M. Bernstein of the University of Pennsylvania in Philadelphia used a bank of 10 computers for 6 months to search the resulting images for faint objects moving in the Kuiper belt.

The study revealed three such objects, the brightest of which was subsequently recorded by the Keck 1 Telescope atop Mauna Kea in Hawaii. Ranging in diameter from 25 to 45 kilometers, the bodies are the smallest objects ever detected at the fringes of the solar system and are one-billionth as bright as the dimmest celestial objects visible to the naked eye. Theoretical predictions about the origin of short-period comets, which traverse the inner solar system, had led Bernstein and his colleagues to expect to find about 85 of these bodies in the Hubble images. At press time, Bernstein was scheduled to report the findings Sept. 6 at a meeting of the American Astronomical Society in Monterey, Calif.

For the Kuiper belt to be the supplier of the short-period comets, it ought to contain hundreds to thousands of times as many bodies as the images suggest, Bernstein's team calculates. A few years ago, Hal F. Levison of the Southwest Research Institute in Boulder, Colo., and a colleague suggested an alternative source for short-period comets. They proposed the group of objects known as the scattered disk, which extends from the outer edge of the classical Kuiper belt.

The new observations suggest that even this population of fringe objects might not be sufficiently massive to spawn the short-period comets, says Bernstein.

His team has proposed an explanation for the dearth of appropriate-size Kuiper belt objects: Many of the larger bodies there may have been shattered into bits by collisions. In fact, other researchers have suggested that the outward migration of Neptune or some other planet early in the history of the solar system triggered collisions within the Kuiper belt.

The new work is "spectacular," says Levison, because it provides the first data on the size of objects in the Kuiper belt. The study not only highlights the uncertainty about the origin of comets but also provides important clues about the formation and evolution of the outer solar system, he says.