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'Whopper' of a Comet Spurs a Festival of Science

Office of News and Public Affairs
University of Wisconsin-Madison

Christopher Anderson, (608) 262-0492, anderson@madraf.astro.wisc.edu
Kenneth H. Nordsieck, (608) 262-1163, khn@sal.wisc.edu
Walter Harris, (608) 265-3436, wharris@harlan.sal.wisc.edu
Frank Scherb, (608) 262-6879, scherb@wisp.physics.wisc.edu
Frederick Roesler, (608) 262-1495, roesler@wisp.physics.wisc.edu


'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

"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 beauty. 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," said Scherb. 
"It's blowing everybody away."


A Lineup of Comet Experiments

As the brightest comet in recent memory makes its pass through the inner
solar system, an unprecedented array of UW-Madison instruments has been
deployed to capture data from this interplanetary voyager.

Four different experiments, at least two of which have never been attempted
before, are underway or in preparation. Here's a rundown of the experiments
and the types of information they might glean from Comet Hale-Bopp.

* Multispectral measurements -- Using one of the most powerful capabilities
of the new WIYN Telescope, a 3.5 meter telescope operated by Wisconsin,
Indiana University, Yale and the National Optical Astronomical Observatories,
astronomer Christopher Anderson is making the first simultaneous multispectral
measurements of the comet and its coma and tail. Employing a device known
as a multi-object spectrograph, capable of making 96 simultaneous
measurements of spectra at different points in space, Anderson can take a
snapshot of the physical chemistry of the comet from the nucleus to regions
deep within the tail.

The idea, Anderson said, is to see what happens and when it happens. Where,
for example, are the various molecular species produced, replaced and
destroyed? "With this approach, we simply get a much more comprehensive
and controlled sample of the comet."

* Imaging -- Some of the most spectacular images of Hale-Bopp are being
captured by the WIYN Telescope. With some of the best imaging capabilities of
any modern telescope and a very wide field of view, WIYN is revealing jets of
material being blown off the nucleus of the comet and showing how the ejected
material moves in space and time.

"There's clearly a jet, and you can clearly see it rotating,"said Walter 
Harris, a UW-Madison astronomer coordinating the four-night WIYN Hale-Bopp
imaging program. "This comet does appear to have one very active (jet)
region" where material may be boiling off from a pocket of ice as the
comet nears the sun.

* Fabry-Perot Spectrometry -- Using novel spectrometers capable of sampling
the faint and diffuse glow of some kinds of light produced by the comet,
physicists Frederick Roesler and Frank Scherb are, among other things,
zeroing in on the spectral signature of water, the prime ingredient of a comet.
"A Fabry-Perot has the advantage of being able to collect and analyze most of
the light that comes from the comet cloud," said Scherb. "Because we can do
this for the long-term and from the ground, we think were going to get a pretty
reliable history of the comet" and its water production.

Another nifty experiment to be conducted by Roesler and Scherb will be to use
the tail of the comet like a windsock. As the comet makes its turn about the 
sun, the UW-Madison physicists may be able to observe the solar wind by
analyzing the behavior of the ion tail of the comet.. In essence, the
comet will act like a free satellite to let scientists sample the solar
wind in regions of interplanetary space where it is difficult to place

* A Rocket -- On March 25, a sounding rocket with the Wide-Field Imaging Survey
Polarimeter or WISP will lift off from a pad at the White Sands Missile Range 
in New Mexico. At the apex of its trajectory 240 miles above the Earth,
WISP will snap a series of eight pictures over the course of about 400
seconds. The pictures, taken in ultraviolet light, will help astronomers
Walter Harris and Kenneth Nordsieck fathom some of the geometric
properties of the dust particles and the carbon-containing ices streaming
off the comet.

"No one has ever done ultraviolet polarimetry on comets," said Harris. "It 
can tell us a great deal about structures in comets, particularly dust. It
tells you a lot about their shape and size."