The Starbase One Astronomy & Space Collection/STARBASE_ONE.ISO/hst/93_22.txt

Text File | 1996-01-12 | 8KB | 166 lines

CONTACT:  Ray Villard, STScI  FOR RELEASE: October 18, 1993
          [phone]
                              PRESS RELEASE NO.: STScI-PR93-22
          Dr. Harold Weaver
          [phone]



       HUBBLE INVESTIGATES COMET ON A COLLISION COURSE WITH JUPITER

NASA's Hubble Space Telescope has provided the most detailed look
yet at the comet hurtling toward a July 1994 collision with the
giant planet Jupiter.

Hubble's high resolution shows that the approximately 20 objects
that comprise comet P/Shoemaker-Levy 9 -- giving it the
resemblance of a "string of pearls" -- are much smaller than
originally estimated from observations with ground-based
telescopes.  According to Dr. Harold Weaver of the Space
Telescope Science Institute (STScI) Baltimore, MD, the Hubble
observations show that the cometary nuclei are probably no bigger
than three miles across, as opposed to earlier estimates of nine
miles.

The new Hubble data show that the impacts will unleash only
1/10th to 1/100th as much energy as thought previously.  However,
even with these new size estimates, the total energy of the
collisions will be equivalent to 100 million megatons of TNT --
10,000 times the total destructive power of the world's nuclear
arsenal (at the peak of the Cold War).  The impacts will be
comparable in strength to the collision of a large asteroid or
comet with Earth 65 million years ago. This latter cosmic catastrophe
is suspected to have caused the extinction of the dinosaurs and
hundreds of other species between the Cretaceous and Tertiary ages.

Weaver and a team of co-investigators are announcing their
analysis of the Hubble observation at the 25th annual meeting of
the Division for Planetary Sciences of the American Astronomical
Society held in Boulder, Colorado.

Since the comet's discovery last March, there have been widely
varying estimates of how energetic the collisions with Jupiter
will be. The force of the collision depends not only on the speed
of the impacting bodies, but their size as well.

Measuring the sizes of the nuclei is very difficult because each
nucleus is surrounded by a haze of dust, called a coma.  "Most of
the light being observed is due to scattering by dust in the
coma," Weaver says.  Relative to ground-based images, the Hubble
image provides improved contrast between the nuclei and their
comae, thereby allowing a better estimate for the sizes.
However, "even the current Hubble image does not allow a clear
separation of nucleus and coma, so its size estimates are still
probably only upper limits to the true nuclear sizes," according
to Weaver.

Fortunately, the definitive answer might be available soon.
During the December Hubble Servicing Mission a new camera called
WFPC-2, (Wide Field and Planetary Camera 2) with corrective
optics to compensate for aberration in Hubble's primary mirror,
will be installed on the telescope. "The Hubble repair should
provide images with much better contrast than the current images,
and if the nuclei are close to the sizes we now think they are,
then they should really pop out in the new Hubble images," Weaver
says.

Hubble's sharp resolution shows that one bright knot in the comet
stream is really four fragments close together.  Two of the
pieces have an apparent separation of only 700 miles.  The Hubble
image also shows that most of the visible nuclei have comparable
sizes.  Weaver says that the close match in size among the chunks
suggests they might be the primordial "building blocks" of
comets. According to calculations, the parent comet broke apart
when it passed close to Jupiter in July 1992.  "Jupiter's gravity
might have disassembled the comet back into the primordial
objects, called planetesimals, that were present when our Sun
formed 4.5 billion years ago," Weaver says.  "However, since the
current Hubble observations cannot detect nuclei much smaller
than about 2 km, the size distribution of the planetesimals is
still indeterminate.  Once Hubble's optics are fixed, we should
get a better handle on the range of sizes within the planetesimal
population."

Though commonly referred to as a comet, some astronomers think
P/Shoemaker-Levy 9 might be an asteroid. In this case, it would
have come from the asteroid belt between the orbits of Jupiter
and Mars, rather than from a hypothetical comet belt beyond
Pluto's orbit.  However, no one has ever seen an asteroid break
apart so it is difficult to predict how asteroids should behave
under these circumstances.  Likewise, since there are few
detailed studies of comets as far away as Jupiter (1/2 billion
miles), it's hard to know how a comet should behave at Jupiter's
distance.

At Jupiter's distance the comet's surface is so cold that the
sublimation rate of water ice is very small. "On the other hand,
the breakup of the comet may have released an unusually large
number of icy grains, exposing such a large surface area to the
sun that the sublimation might become detectable," Weaver says.
"Also, there are probably substances present that are more
volatile than water ice."

Weaver's team took spectra near the brightest fragment to search
for molecules that might have been released from subliming ice.
This would provide strong evidence that P/Shoemaker-Levy 9 is a
comet, not an asteroid.  Spectroscopic observations made with
Hubble Faint Object Spectrograph failed to find hydroxyl
molecules that would be a clear indicator of cometary origin.

Another way to address this mystery is by watching the evolution
of the surrounding coma. A cometary origin would be likely if the
coma is continually replenished by gas streaming off the
fragments, since comets are more icy than asteroids.  However, if
the coma simply spreads out, eventually completely disappearing,
the coma might just be dust from a broken-up asteroid. The
current Hubble image shows that the coma is apparently not
continually being replenished, but more observations are needed
to monitor the coma development further.

The Jupiter collision is expected to occur over a six-day period
around July 21, 1994.  The effect of the impact will depend not
only on the size and velocity of the cometary nuclei, but also
their composition and structure.  Comets are very porous and,
thus, might break up high in the atmosphere.  For example, on
June 30, 1908 a 160-foot (50-meter) wide cometary nucleus or
stony meteor is suspected to have disintegrated in Earth's
atmosphere at an altitude of five miles (8 km).  The resulting
explosion leveled hundreds of thousands of acres of forest in
Siberia's Tunguska River Valley, Russia.

The Jupiter impacts could potentially produce spectacular
phenomena in the giant planet's multicolor cloud tops.  The
plummeting comet nuclei would turn into gigantic versions of
meteors or "shooting stars."  Each 100-mile wide, blue-white
fireball would blow a hole in Jupiter's atmosphere the size of
Texas.  Although the impacts are predicted to occur on Jupiter's
far side (not observable from Earth), it's likely that the
effects on the atmosphere still will be visible as the impact
zone rotates into the Earth's view.  (Jupiter's rotation rate is
9 hours, 50 minutes.)

The Hubble telescope is expected to be a key player during next
year's encounter, although there are no definite observing plans
yet.  Due to its low-level of scattered light and high angular
resolution, the Hubble should be able to observe the comet even
when the glare of Jupiter prevents further ground-based
observations.  After the impact, the Hubble images should show
details in Jupiter's atmosphere that are unattainable by any
other means.

The comet was discovered last March by Dr. Carolyn Shoemaker of
Northern Arizona University, Dr. Eugene Shoemaker of the U.S.
Geological Survey, and veteran amateur comet observer David Levy.
The HST observing team consists of these three people, Weaver,
and 15 others from a variety of institutions.


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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).