Magazine article Science News

What's the Source of Quick-Return Comets?

Magazine article Science News

What's the Source of Quick-Return Comets?

Article excerpt

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. …

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