From: Jan Wee <jwee@mail.arc.nasa.gov>
Subject: Hubble and IUE Hale-Bopp Observations Surprise Astronomers
Date: Thu, 27 Mar 1997 16:17:14 -0600
Dear discuss-lfm members,
For all the Hale-Bopp lovers out there. I saw the comet
last night and it was quite an exciting sight!
Jan Wee, moderator
PRESS RELEASE...
HUBBLE AND IUE HALE-BOPP OBSERVATIONS SURPRISE ASTRONOMERS
Completing an unprecedented year-long study of Comet Hale-
Bopp using two NASA observatories, the Hubble Space Telescope and
the International Ultraviolet Explorer, astronomers report that
they are surprised to find that the different ices in the nucleus
seem to be isolated from each other. They also report seeing
unexpectedly brief and intense bursts of activity from the nucleus
during the monitoring period. The Hubble observations suggest that
the nucleus is huge, 19 to 25 miles across.
The findings, by a team of scientists led by Johns Hopkins
astrophysicist Dr. Harold Weaver, are being published in the March
28 issue of the journal Science.
"Hale-Bopp will probably provide the most revealing portrait
of the workings of a cometary nucleus since the spacecraft missions
to comet Halley in 1986," said Weaver. "This is a unique
opportunity; we have never had the chance to examine a comet in
this much detail, over this large a range of distance from the
Sun."
The key results:
Violent Eruptions on the Comet's Surface
During the course of long-term observations, which began in
August 1995, astronomers unexpectedly caught the comet going
through a sudden brief outburst, where, in little more than an
hour, the amount of dust being spewed from the nucleus increased at
least eight-fold. "The surface of Hale-Bopp's nucleus must be an
incredibly dynamic place, with 'vents' being turned on and off as
new patches of icy material are rotated into sunlight for the first
time," Weaver said.
A Complex, Mottled Nucleus
To their surprise, astronomers found that water ice
sublimates (turns directly from a frozen solid into a gas) at a
different rate than the trace ices, implying that those components
are not contained within the water on the comet. This conclusion
is further supported by Hubble data showing that the rate at which
dust left the nucleus was much different than the sublimation rate
of water. This result is contrary to previous models for a comet's
nucleus, which suggest that the trace components, such as carbon
disulfide ice, are contained inside of the most abundant ice on the
comet, frozen water. As water sublimates, the trace components and
dust should be released at similar rates, but this is not what
Hubble observed.
A Monstrous Nucleus
By studying Hubble Space Telescope images, the astronomers
have estimated that its nucleus may be about 19 to 25 miles in
diameter. The average comet is thought to have a nucleus of about
three miles in diameter, or even smaller. The comet or asteroid
that struck the Earth 65 million years ago, possibly causing the
extinction of the dinosaurs, was probably about six to nine miles
across.
Because Hale-Bopp was unusually bright when it was still a
great distance away, well outside the orbit of Jupiter, it has
given scientists their best view ever of the changes in a comet's
nucleus as it gets closer to, and is progressively heated by, the
Sun. Those changes, in turn, provide information about the
composition and structure of comets, which are believed to be
remnants from the formation of the solar system, about 4.6 billion
years ago. Learning more about comets can provide important
information about the materials and processes that formed the solar
system.
The Space Telescope Science Institute is operated by the
Association of Universities for Research in Astronomy, Inc. (AURA),
for NASA, under contract with the Goddard Space Flight Center,
Greenbelt, MD. The Hubble Space Telescope is a project of
international cooperation between NASA and the European Space
Agency (ESA).
- end -
EDITOR'S NOTE: An image to accompany this release is available to
news media representatives by calling the Headquarters Imaging
Branch on 202/358-1900. Photo numbers are:
HST Hale-Bopp (Color) 97-HC-163 (B&W) 97-H-163
Image files in GIF and JPEG format and captions may be accessed on
Internet via anonymous ftp from ftp.stsci.edu in /pubinfo.
GIF JPEG
PRC97-08 Hale-Bopp gif/hb9596.gif jpeg/hb9596.jpg
Higher resolution digital versions (300 dpi JPEG) of the release
photograph are available in /pubinfo/hrtemp: 97-08.jpg (color) and
97-08bw.jpg (black & white).
GIF and JPEG images, captions and press release text are available
via the World Wide Web at
http://oposite.stsci.edu/pubinfo/PR/97/08.html and via links in
http://oposite.stsci.edu/pubinfo/Latest.html or
http://oposite.stsci.edu/pubinfo/Pictures.html.