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FOR IMMEDIATE RELEASEMarch 10, 1998
ASTRONOMERS TRACK DOWN ASTEROIDS IN HUBBLE ARCHIVE
Astronomers have stumbled on an unusual asteroid hunting
ground: the thousands of Hubble Space Telescope images stored in
The hunt, by Robin Evans and Karl Stapelfeldt of NASA's Jet
Propulsion Laboratory in Pasadena, CA, has yielded a sizable
catch of small asteroids - about 100. Their preliminary analysis
suggests that a total population of 300,000 small asteroids --
essentially rocks just over 1 kilometer to 3 kilometers wide
(half a mile to two miles) -- are orbiting between Mars and
Jupiter in a band of space debris known as the main belt.
Currently, there are 8,319 confirmed main belt asteroids whose
orbits have been measured, and about the same number have been
sighted but not confirmed.
The asteroid hunters also were intrigued that they didn't
find evidence of small comets passing near Earth, a finding
announced last year.
Most astronomers stalk the Hubble archive for bigger game,
such as quasars, distant galaxies, and supernovae, but Evans and
Stapelfeldt have discovered that the pursuit of smaller prey such
as asteroids can be equally successful.
Over a three-year period, the two astronomers and their
collaborators have searched through more than 28,000 Wide Field
and Planetary Camera 2 (WFPC2) images, looking for wide, looping
streaks of light, the telescope's tell-tale signatures of
asteroids. Most of the ones they found are too faint to be
observed by current ground-based search programs. Hubble
captures their images purely by accident: nearby asteroids
inevitably wander across the telescope's field of view while
other, higher priority targets are being observed.
"The archive images are distributed fairly evenly across the
sky, so we find asteroids according to both their position in the
sky and their number," Evans said. "As expected, we see the
asteroids concentrated towards the ecliptic plane and we see
small asteroids because they are the most numerous. Small main-
belt asteroids such as these are the ones most likely to evolve
into Earth-crossing asteroids due to encounters with their larger
neighbors. Some of the asteroids in our survey could eventually
migrate toward Earth."
An accurate asteroid census is an important part of
assessing how many of these small bodies there are which could
potentially pose a hazard to Earth. The Hubble archives
represent a newly-tapped information resource which could help
scientists more precisely estimate the risks they pose to Earth.
The Hubble archival data also strongly limit the number of
small comets that could be passing very near Earth, according to
Evans and Stapelfeldt. Last year, Dr. Louis A. Frank of the
University of Iowa in Iowa City, using data from NASA's Polar
spacecraft, reported he found evidence that about a dozen small
comets strike Earth's upper atmosphere each minute. Evans and
Stapelfeldt estimate the such small comets should be bright
enough to produce thousands of detectable trails in the Hubble
archival images, but these were not seen.
The Hubble images capture an asteroid as a long trail
produced by its motion across the camera's field of view. The
trails appear like the streaks of light found on photos taken at
night of speeding cars with their headlights on. In Hubble's
case, asteroid trails show a unique curvature due to the
continuously shifting position of the telescope as it orbits the
Earth. This effect, known as parallax, allowed Evans and
Stapelfeldt to determine distances and sizes for the
asteroids spied by Hubble. A similar parallax effect is the key
to depth perception in human vision: our eyes are set apart so
that we can see three dimensionally.
"Asteroid trails observed by the Hubble telescope are
usually curved because the telescope travels in a curved low-
Earth orbit," Stapelfeldt says. "By precisely measuring the
shape of the trails, we can solve for the distance to each
asteroid at the time it was observed. It isn't possible to do
this using a stationary telescope on the ground."
Finding asteroids isn't what the two astronomers originally
had in mind. As members of the WFPC2 science team, Evans and
Stapelfeldt were examining test images of distant stars and
galaxies to ensure that the new camera was functioning properly.
These were among the first images taken with WFPC2, which had
restored sharp focus to Hubble's images when it was installed in
late 1993. Stapelfeldt's wife, Deborah Padgett (also an
astronomer), pinpointed the first asteroid in 1994 while looking
at images on the couple's home computer. Intrigued, Evans and
Stapelfeldt began combing through more than 1,600 of the
science team's survey photos, finding 12 more asteroids. This
discovery prompted their large-scale search, by eye, of two years
worth of Hubble archival images.
Evans' and Stapelfeldt's initial results are reported in the
February 1998 issue of the research journal Icarus.
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
NOTE TO EDITORS: A photo and caption are available via the World
Wide Web at: http://oposite.stsci.edu/1998/10 or via links in:
GIF and JPEG images are available via anonymous ftp to
oposite.stsci.edu in /pubinfo/gif/9810.gif and