[meteorite-list] Getting to Know Comet Borrelly

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:02:30 2004
Message-ID: <200203181824.KAA22612_at_zagami.jpl.nasa.gov>

http://www.astronomy.com/Content/Dynamic/Articles/000/000/000/795nbiab.asp

Getting to Know Comet Borrelly

At this week's Lunar and Planetary Science Conference, space scientists
presented new insight on Deep Space 1's recent target, Comet Borrelly.

by Vanessa Thomas
astronomy.com
March 16, 2002

On September 22, 2001, NASA's Deep Space 1 probe proved victorious in a
daring adventure when it introduced a comet called Borrelly to the people of
Earth. In a chancy maneuver that day, Deep Space 1 altered our view of
Borrelly as a mysterious, dust-cloaked object to an exciting astrogeology
subject.

Designed not as a science mission but to test new technologies, Deep Space 1
wasn't built for a risky comet rendezvous. In fact, mission team members
weren't sure whether the spacecraft would even be able to find the comet to
take any pictures at all. But when the encounter was over, scientists had
their most detailed images of a comet's nucleus and the best opportunity yet
to learn about the structure and history of comets.

Less than six months later, on Wednesday, March 13, scientists at the 33rd
Lunar and Planetary Science Conference presented the first round of science
results from Deep Space 1's successful meeting with Comet Borrelly. In
addition to Borrelly's spiky jets, dusty environment, fragmented ice,
solar-wind interaction, and composition, presenters discussed the comet's
varied and fascinating nucleus.

>From their first look at Deep Space 1's pictures of Comet Borrelly,
planetary scientists knew its nucleus was one of the darkest objects they'd
ever seen - a surprising achievement for something composed largely of
bright ice. Overall, the comet's nucleus reflects no more than 3 percent of
the light that hits it.

"The average geometric albedo is black, black, black, and ranges from
extremely black to very black," said University of Tennessee geologist Dan
Britt, who described the nucleus's color as being like that of photocopier
toner. Amidst Borrelly's gloominess, however, Britt managed to identify four
main types of landforms on the strange mini-world.

Mottled terrain covers the extended comet's far opposite ends, which are
darker than Borrelly's mid-section. These rough areas are filled with
bowl-shaped depressions, hills, streaks aligned with the nucleus's long
axis, and notable variations in brightness. The ends lack any strong
association with the comet's jets.

Among the mottled terrain at the ends of the comet's nucleus are spots
darker than anything else on Borrelly. These black patches reflect just 0.7
to 1 percent of incoming light, Britt reported.

The middle of the nucleus consists of Borrelly's brightest and smoothest
terrain. It also contains the comet's fourth type of feature: dark,
flat-topped mesas surrounded by bright slopes. According to Britt, the
smooth terrain and slopes probably reveal the comet's freshest material and
are the likeliest source of its jets.

At the end of his talk, Britt pointed to another curious characteristic on
Comet Borrelly's nucleus. Several ridges and fractures cut across the
comet's slimmest section, which separates its two lobes. Another presenter,
Jürgen Oberst of Germany's DLR Institute of Space Sensor Technology and
Planetary Exploration, added that a three-dimensional stereo elevation map
of the nucleus shows that the smaller end of the bowling-pin-shaped nucleus
is not aligned with but tilted (by 30 degress to 50 degrees) relative to the larger half.
Britt and Oberst suggest these observations indicate Borrelly's nucleus may
actually be a contact binary - composed of two pieces loosely bound to each
other.
Received on Mon 18 Mar 2002 01:24:59 PM PST