An unprecedented yearlong series of observations with the Hubble Space Telescope has revealed a few surprises about comet Hale-Bopp.
The comet thrilled astronomers as it displayed unusual outbursts of increased brightness. By using a spectrograph to analyze ultraviolet light from the comet, scientists learned that the components of its frozen solid nucleus apparently are arranged differently than astronomers would have expected.
The sheer size of the nucleus also makes Hale-Bopp something of a novelty, said Hopkins astrophysicist Hal Weaver, who led the team of scientists conducting the observations.
By studying images taken with the Hubble telescope, the astronomers have estimated that its nucleus may be 30 to 40 kilometers (about 19 to 25 miles) in diameter. In comparison, the potato-shaped nucleus of Halley's comet, considered to be a large one, had an equivalent diameter of about 10 kilometers (6 miles).
To put Hale-Bopp's girth into cosmic perspective, the comet or asteroid that struck the Earth 65 million years ago, possibly causing the extinction of the dinosaurs, probably was about 10 to 15 kilometers (6 to 9 miles) across, Weaver noted.
The findings were detailed in a scientific paper published last Friday in the journal Science, just in time for the comet's rendezvous with the sun on April 1, when it peaks in brightness.
Astronomers studied ultraviolet light from Hale-Bopp with Hubble and the International Ultraviolet Explorer satellite. They observed the comet over a one-year period beginning in late August 1995, when it was about 586 million miles from Earth and 633 million miles from the sun.
Never before had astronomers been able to carefully observe a comet as it approached the sun from such a great distance.
It was a sort of last gasp for the IUE, which was then shut down after 18 years of operation. The Science paper was co-authored by Weaver, Hopkins astrophysicist Paul Feldman and eight scientists from other institutions in the United States and Europe.
Hale-Bopp represented a truly unique opportunity for astronomers because it was unusually bright when it was still far from the sun, well outside the orbit of Jupiter. Meanwhile, astronomers had the perfect tool with which to observe the cometary spectacle: the Hubble Space Telescope. Hubble gave astronomers 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 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. So, learning more about comets can provide important information about the materials and processes that formed the solar system.
The Hubble telescope caught Hale-Bopp going through a sudden brief outburst of brightening in September 1996. In little more than an hour the amount of dust being spewed from the nucleus increased at least eightfold. But then it quickly returned to its normal brightness.
"Comets sometimes have temporal variation," Weaver said. "But this just had huge outbursts."
The sudden brightening suggested that, as the comet rotated, different patches of icy material were heated by the sun, turning on new "vents" as the ice went directly from a solid to a gas, a process known as sublimation, Weaver said.
Astronomers noticed another unusual behavior.
The solid center of a comet is a mushy mixture of dust and frozen chemical compounds, the most abundant of which is water ice. As it gets closer and closer to the sun, the sublimation increases, releasing more gas and dust and producing a long tail that reflects sunlight and brightens the comet.
Scientists had expected the changing rates of frozen water sublimation to govern the changing rates of other frozen chemical compounds, such as carbon dioxide and carbon disulfide. That's because those trace components, along with dust, were generally believed to be contained within frozen water.
Therefore, the changing sublimation rate of carbon dioxide and other components should correspond with the changing rate of water sublimation.
However, after studying the comet in detail as it got closer and closer to the sun, astronomers found that those other components may be arranged in a more complicated manner than previously thought.
To their surprise, astronomers found that the changing sublimation rates of the different frozen compounds did not match the changing rate of water sublimated. The various rates of dust being released from the nucleus also did not match the changing rate of water sublimation, further supporting the finding.
"They seem to have their own minds," Weaver said.
While the vaporization rate of water ice increased more than 13-fold between April and October 1996, there was only a two-fold increase in the rate of dust being released. The vaporization of carbon disulfide ice increased less than threefold.
Perhaps the rates do not match because the different materials are contained in separate regions of the nucleus.
"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, Weaver said.
The comet was discovered in July 1995 by amateur stargazers Alan Hale, of Cloudcroft, N.M., who has a Ph.D. in astronomy, and Thomas Bopp, of Glendale, Ariz.
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