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Astronomers Track Down Asteroids In Hubble Archive

Don Savage
Headquarters, Washington, DC                   March 9, 1998
(Phone:  202/358-1547)

Diane Ainsworth
Jet Propulsion Laboratory, Pasadena, CA
(Phone:  818/354-5011)

Donna Weaver
Space Telescope Science Institute, Baltimore, MD
(Phone:  410/338-4493)

RELEASE.:  98-39

ASTRONOMERS TRACK DOWN ASTEROIDS IN HUBBLE ARCHIVE

       Astronomers have stumbled on an unusual asteroid hunting 
ground:  the thousands of images stored in the Hubble Space 
Telescope archive.

       The hunt, by Robin Evans and Karl Stapelfeldt of NASA's 
Jet Propulsion Laboratory, 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 half a mile to two 
miles wide (1-3 kilometers) -- 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.

        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."  

	The Hubble archives represent a newly tapped information 
resource which could help scientists more precisely estimate 
the risks the asteroids pose to Earth.

        According to Evans and Stapelfeldt, the Hubble archival 
data also strongly limit the number of small comets that could 
be passing very near Earth.  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.

       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 Agency (ESA).

                             - end -

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