NASA's Hubble Space Telescope has detected a long-sought population of comets dwelling at the icy fringe of the solar system. The observation, which is the astronomical equivalent to finding the proverbial needle-in-a-haystack, bolsters proof for a primordial comet reservoir just beyond Neptune, currently the farthest planet from the Sun.

Based on the Hubble observations, astronomers estimate the belt contains at least 200 million comets that have remained essentially unchanged since the birth of the solar system 4.5 billion years ago. The region is thought to be the source of the comet that struck Jupiter in July 1994.

"For the first time, we have a direct handle on the population of comets in this outer region. We now know, conclusively, that our solar system doesn't end at Neptune. The solar system just got a lot more interesting," says Anita Cochran of the University of Texas, Austin. "We now know where these short-period comets formed, and we now have a context for their role in the solar system's evolution."

The existence of a comet-belt encircling our solar system -- like the rings that wrap around Saturn -- was first hypothesized more than 40 years ago by astronomer Gerard Kuiper. The so-named Kuiper Belt remained theory and conjecture until 1992, when ground-based telescopes began detecting about 20 large icy objects ranging from 96 to 320 kilometers (60 to 200 miles) in diameter. However, researchers had to wait for Hubble's high spatial resolution and sensitivity before they could search for an underlying population of much smaller bodies assumed to be present -- just as there are more pebbles than boulders on the beach. The planet Pluto is considered by astronomers to be the largest member of the Kuiper Belt region.

"This is a striking example of what Hubble can do well," says Cochran. "We can at last identify small comet-sized objects that are just a few miles across, about the size of New York's Manhattan Island."

The team believes this apparently closes the mystery of the source of the short-period comets that orbit the Sun in less than 200 years, including such members as comet Encke, Giacobini-Zinner, and the infamous comet Shoemaker-Levy 9 that collided with the planet Jupiter in July 1994. The comet-disk lies just beyond Neptune and might stretch 500 times farther from the Sun than Earth. This is 100 times closer to Earth than the hypothesized Oort Cloud, commonly thought to be a vast repository of comets that was tossed out of the early solar system. Despite the close proximity of the Kuiper Belt comets, experts say they don't pose any greater threat to colliding with Earth than comets that come from much farther out.

"The Kuiper Belt is the best laboratory in the solar system for studying how planets formed," says co-investigator Hal Levison of the Southwest Research Institute's Boulder, Colorado, office. "We believe we are seeing a region of the solar system where the accumulation of planets fizzled out."

The icy nuclei are too far away to have the characteristic shell (coma) and tail of gases and dust that are a comet's trademarks, when it swings close enough to the Sun to warm up and sublimate. Detecting these bodies in their "deep-freeze" state, at the dim horizon of the solar system, pushed Hubble Space Telescope to its performance limits. "Imagine trying to see something the size of a mountain, draped in black velvet, located four billion miles away," says co-investigator Alan Stern, also of the Southwest Research Institute's Boulder office.

The team, consisting of Cochran, Levison, Stern and Martin Ducan of Queen's University, Ontario, Canada, used Hubble's Wide Field Planetary Camera 2 (WFPC2) to observe a selected region of the sky, in the constellation Taurus, that had few faint stars and galaxies that would confuse the search.

The team plans to continue searching for more objects. They have already collected more images with Hubble that allow them to better quantify the number and sizes of comets in the Kuiper Belt.

The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) for NASA, under contract with the Goddard Space Flight Center, Greenbelt, Maryland. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency (ESA).

Don Savage
Headquarters, Washington, DC                June 14, 1995
(Phone:  202/358-1547)

Tammy Jones
Goddard Space Flight Center, Greenbelt, Maryland
(Phone:  301/286-5566)

Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
(Phone:  410/338-4514)

RELEASE:  95-88

Comet Introduction Kuiper Belt Objects

Calvin J. Hamilton