'Whopper' Of A Comet Spurs A Festival Of Science

Comet Hale-Bopp, speeding toward an April 1 rendezvous with the sun, is becoming everything a cautious community of astronomers had hoped: a brilliant object whose brightness and size will enable scientists to dissect a comet like never before.

With an arsenal of new observational tools, and enough forewarning to deploy them, astronomers worldwide are engaged in a festival of observations that together promise to reveal new secrets of comets that, like time capsules, harbor clues to the earliest conditions of our solar system.

"It's a whopper," said Kenneth H. Nordsieck, a University of Wisconsin-Madison astronomer helping to ready a rocket payload that, in a brief flight to the top of the Earth's atmosphere later this month, will make the world's first ultraviolet polarimetric images of a comet.

Nordsieck's pictures, like the observations of four other UW-Madison scientists, will be made with tools rarely, and in some cases never, arrayed for the appearance of a comet. Likened to dirty snowballs because they are composed mostly of dust-sprinkled ice, comets spend most of their lives beyond the rim of the solar system in elongated orbits that rarely bring them within reach of Earthbound astronomers.

"We'll know better how to build a picture of a comet," said Christopher Anderson, a UW-Madison astronomer making an unprecedented set of simultaneous spectral measurements with the state-of-the-art WIYN Telescope atop Kitt Peak, Ariz. "What we're doing has never been done before, and this comet is a beaut. We're having a sinful amount of fun."

As comets go, Hale-Bopp is proving to be especially cooperative. By making itself visible more than a year-and-a-half ago, Hale-Bopp gave astronomers ample time to propose and set up programs of observation and experiments that couldn't be done on the short notice that comets typically provide as they race past Earth and the inner solar system. Moreover, Hale-Bopp is big, bright and exhibiting unusual features that may reveal new insights into the composition and behavior of comets.

"The comet has been bizarre from the get-go," said UW-Madison astronomer Walter Harris. "We like bizarre. It tells us new things."

At the UW-Madison, no fewer than four separate observational programs (see related story) are gathering data from the comet, making the effort here one of the most concentrated and diverse on the planet. In addition to the rocket experiment to be launched March 25 at the White Sands Missile Range in New Mexico, and the novel spectral measurements, Wisconsin scientists are engaged in an ambitious imaging program utilizing the 3.5 meter WIYN Telescope, and a trio of spectrometers designed to sample the faint glow of neutral and ionized gas as it streams off the comet.

Combined, all of these new observations will give astronomers the raw material to construct the most intricate portrait of a comet to date.

While astronomers already know a great deal about comets, there are many fine points about their composition and behavior that have yet to be resolved. The answers to those questions might yield new insights into the early solar system since comets are believed to be leftovers after Earth, Mars, Venus and the rest of the planets were formed 4.5 billion years ago. And because comets spend most of their time in the distant and frozen reaches of the outer solar system, far from the effects of the sun, they are thought to contain material that may have changed little since the planets were first formed.

The chemistry of comets, which are composed mostly of water, is complicated.
Sprinkled throughout is a zoo of materials including dust, ammonia ice and carbon compounds like methane, all of which have telltale signatures, and which seem to mutate into different variants as the comet, making its swing through the inner solar system, is subjected to intense radiation from the sun.

Materials containing carbon, for instance, are of interest to scientists from a number of viewpoints: "By looking at cometary carbon, we can try to understand how the gas and dust in interstellar space changes when it condenses into cometary material in the far reaches of the solar system," said Nordsieck. "Some things happen to it, even at very cold temperatures."

As material boils off the comet, it can undergo violent change. Electrons are stripped off of molecules transmuting them into ionized gas. The change experienced by water molecules as they stream off of a comet, for example, is of fundamental interest to scientists.

Using a trio of novel Fabry-Perot spectrometers, two of which are attached to the McMath-Pierce Solar Telescope on Kitt Peak, Ariz., UW-Madison physicists Frederick Roesler and Frank Scherb, can study the faint glow of ionized gas as it streams away from the comet. Using a third Faby-Perot spectrometer known as WHAM for Wisconsin H-Alpha Mapper, they can study that glow over a very wide field of view which is important, said Scherb, "because comets take up a big piece of the sky."

In essence, "what we're studying is one of the most fundamental processes of a comet: how it processes water," said Scherb. One of these processes manifests itself in an "ionized cloud of gas that stretches for millions of miles in the comet's tail."

Using instrumentation that is 50 to 100 times more sensitive, and that has a very wide field of view, said Roesler, will help get at fundamental issues such as how many tons of water are blown off the comet every second.

"Our gut feeling is that this comet is going to be a bonanza,