Completing an unprecedented year-long study of Comet Hale- Bopp using two NASA observatories, the Hubble Space Telescope and the International Ultraviolet Explorer, astronomers report that they are surprised to find that the different ices in the nucleus seem to be isolated from each other. They also report seeing unexpectedly brief and intense bursts of activity from the nucleus during the monitoring period. The Hubble observations suggest that the nucleus is huge, 19 to 25 miles across.

The findings, by a team of scientists led by Johns Hopkins astrophysicist Dr. Harold Weaver, are being published in the March 28 issue of the journal Science.

"Hale-Bopp will probably provide the most revealing portrait of the workings of a cometary nucleus since the spacecraft missions to comet Halley in 1986," said Weaver. "This is a unique opportunity; we have never had the chance to examine a comet in this much detail, over this large a range of distance from the Sun."

The key results:

Violent Eruptions on the Comet's Surface

During the course of long-term observations, which began in August 1995, astronomers unexpectedly caught the comet going through a sudden brief outburst, where, in little more than an hour, the amount of dust being spewed from the nucleus increased at least eight-fold. "The surface of Hale-Bopp's nucleus must be an incredibly dynamic place, with 'vents' being turned on and off as new patches of icy material are rotated into sunlight for the first time," Weaver said.

A Complex, Mottled Nucleus

To their surprise, astronomers found that water ice sublimates (turns directly from a frozen solid into a gas) at a different rate than the trace ices, implying that those components are not contained within the water on the comet. This conclusion is further supported by Hubble data showing that the rate at which dust left the nucleus was much different than the sublimation rate of water. This result is contrary to previous models for a comet's nucleus, which suggest that the trace components, such as carbon disulfide ice, are contained inside of the most abundant ice on the comet, frozen water. As water sublimates, the trace components and dust should be released at similar rates, but this is not what Hubble observed.

A Monstrous Nucleus

By studying Hubble Space Telescope images, the astronomers have estimated that its nucleus may be about 19 to 25 miles in diameter. The average comet is thought to have a nucleus of about three miles in diameter, or even smaller. The comet or asteroid that struck the Earth 65 million years ago, possibly causing the extinction of the dinosaurs, was probably about six to nine miles across.

Because Hale-Bopp was unusually bright when it was still a great distance away, well outside the orbit of Jupiter, it has given scientists their best view ever of the changes in a comet's nucleus as it gets closer to, and is progressively heated by, the Sun. Those changes, in turn, provide information about the composition and structure of comets, which are believed to be remnants from the formation of the solar system, about 4.6 billion years ago. Learning more about comets can provide important information about the materials and processes that formed the solar system.

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, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency (ESA).

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