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
the 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.
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.
Credit: H.A. Weaver (Applied Research Corp.), P.D. Feldman (The Johns
Hopkins University), and NASA
