Comet Has Observers In Heaven -- Hale-Bopp's Outbursts Stun Astronomers

Astronomy. As Comet Hale-Bopp fades from view, scientists are busy analyzing the data they've collected from telescope observations and other means. What they've found has been expected and surprising, adding to clues about the nature and origin of comets.

CAMBRIDGE, Mass. - As Comet Hale-Bopp fades in the southern skies, the prospects for astronomers studying it are brightening.

The Great Comet of 1997 is giving researchers an unprecedented look at the biggest known icy visitor from the outer solar system.

Hale-Bopp currently is visible only in the Southern Hemisphere, and it is dimming from its peak brightness of March and April. But as it sails away from the sun, it leaves behind detailed information on what makes a comet.

"This is a spectacular comet," said astronomer David Jewitt of the University of Hawaii. "It has let us measure all sorts of things that weren't possible before."

Those new measurements include the many chemicals making up the tails and icy heart of the comet, the spin and size of its giant nucleus and the bright jets of gas and dust that spew outward.

Details on the comet's inner structure may also give scientists a glimpse of the big picture, of what the early solar system was like. Comets are thought to have formed from the same primitive stuff that the solar system did billions of years ago.

"Comets are like an archaeological study of what happened in the past," said Karen Meech, also a University of Hawaii astronomer, who presented some of the latest results at a recent planetary science meeting in Cambridge.

To catch a comet

But getting a good comet to study can be as hard as conducting an archaeological dig in downtown Manhattan. Only a few comets, such as Comet Halley, are known to swing by the sun twice in a person's lifetime. Many previously unknown comets appear with little warning, then disappear again for thousands of years.

Hale-Bopp, for instance, last passed through the inner solar system about 4,200 years ago. It will next visit the area in about 2,380 years, astronomers believe; the difference comes from its being tugged around by the gravity of planets, especially Jupiter, on this pass through the solar system.

Hale-Bopp was discovered in July 1995, so astronomers had nearly two years to prepare for it. Compared with that, 1996's great comet, Hyakutake, was "a flash in the pan," said Dr. Jewitt. Hyakutake was discovered only two months before its closest approach.

But Hyakutake came much closer to the sun, which was the main reason it appeared so bright. Hyakutake came within 10 million miles of the sun, compared with Hale-Bopp's closest approach of 120 million miles.

Hale-Bopp nevertheless managed to appear bright because its nucleus was unusually large - about 25 miles across, compared with 5 or so miles for a typical comet. Because Hale-Bopp was so big, scientists using telescopes could see it way out beyond the orbit of Jupiter. Astronomers were able to track changes in its nucleus and tails as it approached the sun.

They discovered that Hale-Bopp was by far the most active comet ever observed.

"The important thing about Hale-Bopp is that it was producing the most gas and dust of any comet in modern times," said Michael A'Hearn, a comet expert at the University of Maryland in College Park.

That meant scientists could make observations never possible before. Researchers met in Cambridge recently to discuss their initial findings at a meeting of the American Astronomical Society's Division for Planetary Sciences.

Dust, ice eruptions

Scientists at the meeting said that throughout the second half of 1995, most of the gas venting from Hale-Bopp's icy nucleus was carbon monoxide. That's typical of many comets, whose cores are made up of frozen ices including carbon monoxide, water and other compounds. Although Hale-Bopp's nucleus was spewing out enormous amounts of carbon monoxide in one giant jet, it otherwise seemed to be a pretty ordinary comet, A'Hearn said.

But soon, other astronomers saw Hale-Bopp go through an extraordinary outburst. Harold Weaver, an astronomer at Johns Hopkins University in Baltimore, and colleagues watched the comet's nucleus with the Hubble Space Telescope and noticed that the amount of dust coming from the nucleus increased eightfold in little more than an hour.

Hale-Bopp then settled down to relatively normal activity. But when the comet got within about 300 million miles of the sun, it began to change dramatically, Weaver said. Instead of carbon monoxide, frozen water began evaporating off the comet's nucleus.

At the same time, Weaver's team, along with other astronomers using many different telescopes, watched the nucleus of Hale-Bopp sprout five or six distinct jets of gas and dust. Suddenly, the normally roundish nucleus looked like a porcupine sprouting spines.

"It sprung a bunch of leaks," said Weaver. "That's when it really took off."

Eventually, the outflow of water from Hale-Bopp peaked at about 50 million tons coming off each day - by far the most activity ever seen in a comet, said Michael Combi of the University of Michigan.

As the comet got even closer to the sun in the spring the jets coming from the nucleus began to appear as layered arcs surrounding the comet's head. Astronomers think the arcs formed as the jets turned on and off while the nucleus rotated every 11.5 hours. But they don't understand the complete dynamics of how the arcs formed.

"There's something strange going on here," said A'Hearn.

A team of French astronomers from the Observatoire de Besancon found similar arcs in Hyakutake and Halley, they reported at the Cambridge meeting.

As the arcs were appearing around Hale-Bopp's nucleus, an invisible cloud of hydrogen gas also began to grow around the comet.

At its greatest extent, the cloud stretched more than 60 million miles across. That made it more than 70 times the size of the sun and 10 times the size of the hydrogen cloud that surrounded Comet Hyakutake.

Sodium emissions

Hale-Bopp also was emitting huge amounts of sodium. Astronomers were able for the first time to take a picture of a sodium tail trailing behind a comet. The sodium tail streams behind the comet like the two other tails, one made of gas and one of dust.

"We've seen sodium in comets for many years, but we've never taken a picture like this," said astronomer Jody Wilson of Boston University, a member of one of the first teams to take a picture of the sodium tail.

And there's a lot of sodium there. At its peak, Hale-Bopp was producing sodium at a rate comparable to Jupiter's moon Io, previously considered the most productive source of sodium in the solar system.

The amount of sodium in Hale-Bopp's tail actually increases at a greater distance from the nucleus, said Wilson. That could mean the sodium came from chemical reactions within the gas tail itself.

Sodium is one of many compounds seen in comets for years; others include carbon monoxide, methane and ammonia. In Hale-Bopp, however, astronomers have detected a number of other compounds for the first time, including formic acid and sulfur dioxide, said Dominique Bockelee-Morvan of the Observatoire de Paris in France.

Although scientists don't expect to discover new molecules in Hale-Bopp as it travels out of the solar system, they are continuing to study it in hopes of seeing something unusual.

A team of scientists led by Alan Stern of the Southwest Research Institute in Boulder, Colo., recently took about 250,000 ultraviolet pictures of Hale-Bopp from the space shuttle Discovery. Stern plans to patch the pictures into a video to study changes in the comet's shape over time.

The comet will remain visible to the naked eye in the Southern Hemisphere, perhaps through November.

And in the Northern Hemisphere, the comet will become barely visible again to viewers south of 45 degrees latitude - an imaginary line running through Maine. From about mid-September to mid-October, the comet will appear low and extremely dim above the southeastern horizon. ----------------------------------------------------------------- On the Web

-- For scientific results from Hale-Bopp, see http://encke.jpl.nasa.gov.

-- For more pictures of and information on the sodium tail, see http://vega.bu.edu

-- For space-shuttle observations, see http://www.boulder.swri.edu/swuis