November 15, 2007

PHOTO NO.: STScI-PRC07-40

HUBBLE ZOOMS IN ON HEART OF MYSTERY COMET

NASA's Hubble Space Telescope has probed the bright core of Comet
17P/Holmes, which, to the delight of sky watchers, mysteriously
brightened by nearly a millionfold in a 24-hour period beginning
Oct. 23, 2007.

Astronomers used Hubble's powerful resolution to study Comet Holmes'
core for clues about how the comet brightened. The orbiting
observatory's Wide Field Planetary Camera 2 (WFPC2) monitored the comet
for several days, snapping images on Oct. 29, Oct. 31, and Nov. 4.
Hubble's crisp "eye" can see objects as small as 33 miles (54
kilometers) across, providing the sharpest view yet of the source of the
spectacular brightening.

The Hubble image at right, taken Nov. 4, shows the heart of the comet.
The central portion of the image has been specially processed to
highlight variations in the dust distribution near the nucleus. About
twice as much dust lies along the east-west direction (the
horizontal direction) as along the north-south direction (the vertical
direction), giving the comet a "bow tie" appearance.

The composite color image at left, taken Nov. 1 by an amateur
astronomer, shows the complex structure of the entire coma, consisting
of concentric shells of dust and a faint tail emanating from the comet's
right side.

The nucleus-the small solid body that is the ultimate source of all the
comet's activity-is still swaddled in bright dust, even 12 days after
the spectacular outburst. "Most of what Hubble sees is sunlight
scattered from microscopic particles," explained Hal Weaver of The Johns
Hopkins University Applied Physics Laboratory in Laurel, Md., who led
the Hubble investigation. "But we may finally be starting to detect the
emergence of the nucleus itself in this final Hubble image."

Hubble first observed Comet 17P/Holmes on June 15, 1999, when there was
virtually no dusty shroud around the nucleus. Although Hubble cannot resolve
the nucleus, astronomers inferred its size by measuring its brightness.
Astronomers deduced that the nucleus's diameter was approximately 2.1 miles
(3.4 kilometers), about the length of New York City's Central Park. They
hope to use the new Hubble images to determine the size of the comet's
nucleus to see how much of it was blasted away during the outburst.

Hubble's two earlier snapshots of Comet Holmes also showed some
interesting features. On Oct. 29, the telescope spied three "spurs" of
dust emanating from the nucleus, while the Hubble images taken on
Oct. 31 revealed an outburst of dust just west of the nucleus.

The Hubble images, however, do not show any large fragments near the
nucleus of Comet Holmes, unlike the case of Comet
73P/Schwassmann-Wachmann 3 (SW3). In the spring of 2006 Hubble
observations revealed a multitude of "mini-comets" ejected by SW3 after
the comet increased dramatically in brightness.

Ground-based images of Comet Holmes show a large, spherically
symmetrical cloud of dust that is offset from the nucleus, suggesting
that a large fragment did break off and subsequently disintegrated into
tiny dust particles after moving away from the main nucleus.

Unfortunately, the huge amount of dust near the comet's nucleus and the
comet's relatively large distance from Earth (149 million miles, or 1.6
astronomical units, for Holmes versus 9 million, or 0.1 astronomical
unit for SW3), make detecting fragments near Holmes nearly impossible
right now, unless the fragments are nearly as large as the nucleus
itself.

The Hubble Comet Holmes observing team comprises H. Weaver and C. Lisse
(The Johns Hopkins University Applied Physics Laboratory); P. Lamy
(Laboratoire d'Astrophysique de Marseille, France); I. Toth (Konkoly
Observatory, Hungary); M. Mutchler (Space Telescope Science Institute);
W. Reach (California Institute of Technology); and J. Vaubaillon
(California Institute of Technology).

Credit for Hubble image: NASA, ESA, and H. Weaver (The Johns Hopkins
University Applied Physics Laboratory)

Credit for ground-based image: A. Dyer, Alberta, Canada

For images and more information about Comet 17P/Holmes, visit:

http://hubblesite.org/news/2007/40
http://www.jhuapl.edu/newscenter/
http://www.spacetelescope.org/news/html/heic0718.html

For additional information, contact:
Donna Weaver/Ray Villard
Space Telescope Science Institute, Baltimore, Md.
1-410-338-4493/4514
dweaver@stsci.edu/villard@stsci.edu

Mike Buckley
The Johns Hopkins University Applied Physics Laboratory, Laurel, Md.
1-240-228-7536
mike.buckley@jhuapl.edu

Dr. Hal Weaver
The Johns Hopkins University Applied Physics Laboratory, Laurel, Md.
1-443-778-8078
hal.weaver@jhuapl.edu

The Hubble Space Telescope is a project of international cooperation
between NASA and the European Space Agency. The Space Telescope Science
Institute conducts Hubble science operations. The institute is operated
for NASA by the Association of Universities for Research in Astronomy,
Inc., Washington.

The Applied Physics Laboratory, a not-for-profit division of The Johns
Hopkins University, meets critical national challenges through the
innovative
application of science and technology. For more information, visit
www.jhuapl.edu