Volume 5, Number 3     May/June 1997

  Moving Forward

Hale-Bopp Observations Surprise Astronomers

N UNPRECEDENTED YEAR-LONG STUDY of Comet Hale-Bopp, using two NASA observatories—the Hubble Space Telescope and the International Ultraviolet Explorer—has led to some surprising findings. Astronomers found that different ices in the nucleus seem to be isolated from each other. They also reported seeing unexpectedly brief and intense bursts of activity from the nucleus. The Hubble observations suggested the nucleus is huge (19 to 25 miles across).

"Hale-Bopp will probably provide the most revealing portrait of the workings of a cometary nucleus since the spacecraft missions to Halley's Comet in 1986," said Johns Hopkins astrophysicist Dr. Harold Weaver, who led a team of scientists who published their findings in a recent issue of Science. "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."

Astronomers unexpectedly caught the comet going through a sudden brief outburst during the observations that began in August 1995. The dust spewed from the nucleus increased at least eight-fold in just over an hour. "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.

Astronomers also found that water ice turns directly from a frozen solid into a gas at a different rate than trace ices, implying that those components are not contained within water on the comet. This conclusion is further supported by Hubble data showing the rate at which dust left the nucleus was much different than the sublimation rate of water. Astronomers viewing the comet's images from Hubble estimate its nucleus at 19 to 25 miles in diameter. The average comet's nucleus is no more than about three miles in diameter. The comet or asteroid that struck Earth 65 million years ago, possibly causing the dinosaurs' extinction, was about six to nine miles across.

Hale-Bopp was unusually bright when it was well outside Jupiter's orbit, giving scientists their best view ever of the changes in a comet's nucleus as it gets closer to the Sun. Those changes provide information about the composition and structure of comets, which are believed to be remnants of the Solar System's formation. Learning more about comets could provide important information about the materials and processes that formed the Solar System.

This is a series of Hubble Space Telescope observations of the region around the nucleus of Hale-Bopp.

For more information, contact Dr. Harold Weaver at Johns Hopkins University. Call 410/516-7343.
Or contact Donald Savage at NASA Headquarters. Call 202/358-1547, E-mail: dsavage@hq.nasa.gov
Please mention you read about it in Innovation.