The Starbase One Astronomy & Space Collection/STARBASE_ONE.ISO/a96/disk12/95_26.txt

Text File | 1996-01-19 | 5.4 KB | 113 lines

EMBARGOED UNTIL 11:00 A.M. EDT JUNE 14, 1995

CONTACT:  Ray Villard
          Space Telescope Science Institute, Baltimore, MD
          (Phone:  [phone])

          Anita Cochran
          University of Texas, Austin, TX
          (Phone:  [phone])

          Hal Levison and Alan Stern
          Southwest Research Institute, Boulder, CO
          (Phone:  [phone])

          Martin Duncan
          Queen's University, Ontario, Canada
          (Phone:  [phone])


HUBBLE IDENTIFIES A LONG-SOUGHT POPULATION
OF COMETS BEYOND NEPTUNE

NASA's Hubble Space Telescope has detected a long-sought population of
comets dwelling at the icy fringe of the solar system.  The
observation, which is the astronomical equivalent to finding the
proverbial needle-in-haystack, bolsters proof for a primordial comet
reservoir just beyond Neptune, currently the farthest planet from the
Sun.

Based on the Hubble observations, a team of astronomers consisting of
Anita Cochran of the University of Texas, Austin, TX, Hal Levison and
Alan Stern of Southwest Research Institute, San Antonio, TX branch
office in Boulder, CO, and Martin Duncan of Queen's University,
Ontario, Canada, estimate the belt contains at least 200 million
comets, which have remained essentially unchanged since the birth of
the solar system 4.5 billion years ago.

"For the first time, we have a direct handle on the population of
comets in this outer region.  The solar system just got a lot more
interesting," said Cochran.  "We now know where these short-period
comets formed, and we now have a context for their role in the solar
system's evolution."

The existence of a comet-belt encircling our solar system -- like the
rings which wrap around Saturn -- was first hypothesized more than 40
years ago by astronomer Gerard Kuiper. The so-named Kuiper Belt
remained theory and conjecture until 1992, when ground-based telescopes
began detecting about 20 large icy objects ranging from 60 to 200 miles
in diameter.  The planet Pluto is considered by astronomers to be the
largest member of the Kuiper Belt region.  However, researchers had to
wait for Hubble Space Telescope's high spatial resolution and
sensitivity before they could search for an underlying population of
much smaller bodies assumed to be present -- just as there are more
pebbles on the beach than boulders.

"This is a striking example of what Hubble can do well," said Cochran.
"We can at last identify small comet-sized objects that are just a few
miles across, about the size of New York's Manhattan Island.  "Cochran
discussed her team's findings at a 11:00 a.m. news conference June 14,
at the 186th meeting of the American Astronomical Society in
Pittsburgh, PA.

The team believes this apparently closes the mystery of the source of
the short period comets, that orbit the Sun in less than 200 years,
including such members as comet Encke, Giacobini-Zinner, and the
infamous comet Shoemaker-Levy 9 that collided with the planet Jupiter
in July, 1994.  The comet-disk lies just beyond Neptune and might
stretch 500 times farther from the Sun than Earth.  This is 100 times
closer to Earth than the hypothesized Oort cloud, commonly thought to
be a vast repository of comets that were tossed out of the early solar
system.  Despite their close proximity, the Kuiper belt comets don't
pose any greater threat of colliding with Earth than comets that come
from much farther out, said experts.

The comet nuclei are the primordial building blocks that condensed out
of the cloud of gas, dust and ices that collapsed to form the Sun.
"Knowing where comets come from will help constrain models for the
formation of the solar system and tells us something new about where we
came from," Cochran emphasized.

"The Kuiper Belt is the best laboratory in the solar system for
studying how planets formed," said Levison.  "We believe we are seeing
a region of the solar system where the accumulation of planets fizzled
out."

The icy nuclei are too far away to have the characteristic shell (coma)
and tail of gasses and dust that are a comet's trademarks, when it
swings close enough to the Sun to warm up and sublimate.  Detecting
these bodies in their "deep-freeze" state, at the dim horizon of the
solar system, pushed Hubble Space Telescope to its performance limits.
"Imagine trying to see something the size of a mountain, draped in
black velvet, located four billion miles away," said Stern.

The team used Hubble's Wide Field Planetary Camera 2 (WFPC 2) to
observe a selected region of the sky in the constellation Taurus, that
had few faint stars and galaxies that would confuse the search.  The
detection is based purely on a statistical approach, because the
objects being discovered are so faint.

The team plans to continue searching for more objects.  They have
already collected more images with Hubble.  These additional images
allow them to better quantify the number and sizes of comets in the
Kuiper belt.  They also will apply for more Hubble observing time in
the future to probe the structure of the Kuiper belt.

                                * * * * *

The Space Telescope Science Institute is operated by the Association
of Unviersities 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).