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by Harold F. Levison, PhD.
N
ASA's Hubble Space Telescope has
helped scientists detect a long-sought
population of comets dwelling at the icy
fringe of the solar system. The observation, which is the astronomical equivalent of finding the proverbial needle in
the haystack, bolsters proof for a primordial comet reservoir just beyond
Neptune, at present the planet farthest
from the Sun.
Based on the Hubble observations,
a team of astronomers including principal investigator Anita Cochran of the
University of Texas in Austin; the author
and Dr. Alan Stern of Southwest
Research Institute's Boulder, Colorado,
office; and Martin Duncan of Queen's
University, Ontario, Canada, estimate
the belt contains at least 200 million
comets - the remnants of the birth of
Above is an artist's concept of the Kuiper Belt of comets
outside Neptune 's orbit (planets are not drawn to scale).
The comets are debris remaining from the formation of
the solar system 4.5 billion years ago.
our solar system 4.5 billion years ago.
The region is thought to be the source of
comet Shoemaker-Levy 9.
"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," Cochran says. "We
now know where these short-period
comets formed, and we 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
that wrap around Saturn - was hypothesized more than 40 years ago by
astronomer Gerard Kuiper. The so-named
Kuiper Belt remained just a theory until
ground-based telescopes began detecting
about 30 large icy objects ranging from 60
to 200 miles in diameter. The planet Pluto,
Comet Reservoir Detected
at Outer Reaches of
Solar System
Hubble Telescope helps astronomers
confirm existence of Kuiper Belt
10
Technology Today· Fall 1995
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with a radius of 1,200 km, is considered
by astronomers to be the largest member
of the Kuiper Belt region. However,
researchers had to wait for the Hubble
Space Telescopes' 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
the Hubble can do well," continues
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." The team discussed
their findings at the 186th meeting of the
American Astronomical Society in
Pittsburgh, Pennsylvania, in June 1995.
The findings apparently close the
mystery of the source of 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, which
collided with Jupiter in July 1994. The
comet belt lies just beyond Neptune and
may 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 planetary system. Despite their
close proximity to the sun, the Kuiper
belt comets don't pose any greater threat
of colliding with Earth than comets that
come from much farther out.
Senior Research Scientist Or. Harold F,
Levison works in the Space Sciences
Department Boulder, Colorado, office,
where he specializes in celestial mechanics
and the origins of the solar system.
Scientists believe the Kuiper Belt is
the best laboratory in the solar system
for studying how the planets formed,
because, unlike the planets, the comets
have remained largely unaltered since
the solar system was born.
Comet nuclei are considered 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 define and develop models for the
formation of the solar system and will
reveal something new about our origins.
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 the Hubble Space
Telescope to its performance limits.
The team used the Hubble's Wide
Field Planetary Camera 2 to observe a
selected region of the sky in the constellation Taurus, which had few faint
stars and galaxies to confuse the
search. The detection is based purely
on a statistical approach, because the
objects being discovered are so faint
(see box below).
The search for objects continues,
and more images have already been collected with the Hubble. These images
will allow scientists to better quantify
the number and sizes of comets in the
Kuiper belt, as will additional Hubble
observations slated for the future . •:.
Observational Techniques
To isolate and subtract the effects of cosmic ray strikes on the Hubble' s
Wide Field Planetary Camera 2 electronic detectors, which can mimic the
faint signature of a comet, 34 images were taken of the same section of sky.
The cosmic ray hits change from picture to picture, but real objects remain
constant. However, pinpointing comets is even trickier, because they drift
slowly along their orbit about the Sun. Although the orbital periods of these
objects are 200 years or longer, the Hubble has sufficient spatial resolution to
see them move in just a few minutes. This means the comets change position
from picture to picture, just as cosmic ray strikes would. However, cosmic
ray strikes are randomly placed events, while the motions of the comets are
well defined.
To distinguish between the comets and cosmic ray effects, the 34
images were digitally shifted and stacked to the predicted offset to account
for the expected drift rate of comets. The approach is similar to having a
fixed camera on a tripod take a rapid series of snapshots of someone walking in front of the lens. The resulting snapshots can be stacked so the person
appears stationary.
The shift-stack technique was further tested by dividing the images into
groups and running an automated search algorithm to look for objec;ts that
showed up in the same position on most sets of exposures.
The researchers tested the reliability of the approach by shifting the
stacked pictures in the opposite direction of expected comet motion. Ideally,
no comets should have appeared, but random alignments added up to 24
anomalous detections.
When the team stack-shifted the pictures in the direction of the predicted comet motion, they came up with 53 objects. Assuming that 24 of
these are statistically anomalous as well leaves a remainder of 29 objects considered "real."
Technology Today. Fall 1995
11