These NASA Hubble Space Telescope pictures of comet Hale-Bopp
show a remarkable "pinwheel" pattern and a blob of free-flying
debris near the nucleus. The bright clump of light along the spiral
(above the nucleus, which is near the center of the frame) may
be a piece of the comet's icy crust that was ejected into space
by a combination of ice evaporation and the comet's rotation,
and which then disintegrated into a bright cloud of particles.
Although the "blob" is about 3.5 times fainter than
the brightest portion at the nucleus, the lump appears brighter
because it covers a larger area. The debris follows a spiral pattern
outward because the solid nucleus is rotating like a lawn sprinkler,
completing a single rotation about once per week. Ground-based
observations conducted over the past two months have documented
at least two separate episodes of jet and pinwheel formation and
fading. By coincidence, the first Hubble images of Hale-Bopp,
taken on September 26, 1995, immediately followed one of these
outbursts and allow researchers to examine it at unprecedented
detail. For the first time they see a clear separation between
the nucleus and some of the debris being shed. By putting together
information from mthe Hubble images and those taken during the
recent outburst using the 82 cm telescope of the Teide Observatory
(Tenerife, Canary Islands, Spain), astronomers find that the debris
is moving away from the nucleus at a speed (projected on the sky)
of about 68 miles per hour (109 kilometers per hour). The Hubble
observations will be used to determine if Hale-Bopp is really
a giant comet or rather a more moderate-sized object whose current
activity is driven by outgassing from a very volatile ice which
will "burn out" over the next year. Comet Hale-Bopp
was discovered on July 23, 1995 by amateur astronomers Alan Hale
and Thomas Bopp. Though this comet is still well outside the orbit
of Jupiter (almost 600 million miles, or one billion kilometers
from Earth) it looks surprisingly bright, fueling predictions
that it could become the brightest comet of the century in early
1997. The full-field picture on the left, taken with the Wide
Field Planetary Camera 2 (in WF mode), shows the comet against
a stellar backdrop in the constellation Sagittarius. The stars
are streaked due to a combination of Hubble's orbital motion and
its tracking of the nucleus, which is now falling toward the Sun
at 33,800 miles per hour (54,000km/hr). In the close-up picture
on the right, the stars have been subtracted through image processing.
Each picture element is nearly 300 miles (480 km) across at the
comet's distance. In this false color scale the faintest regions
are black, the brightest regions are white, and intermediate intensities
are represented by different levels of red. Even more detailed
Hubble images will be taken with the Planetary Camera in late
October to follow the further evolution of the spiral, look for
more outbursts, place limits on the size of the nucleus, and use
spectroscopy to study the enigmatic comet's chemical composition.
m Credit: H.A. Weaver (Applied Research Corp.), P.D. Feldman (The
Johns Hopkins University), and NASA.
