University of Arizona News Services
UA News Services
Stephen M. Larson
May 4, 1998
UA Catalina Sky Survey to make speedy searches for faint near-earth
asteroids and comets
Astronomers at The University of Arizona in Tucson who in 1992 started a
unique near-Earth asteroid survey in the Santa Catalina Mountains north of
Tucson are about to begin faster searches for fainter objects in large areas
of sky where other such surveys seldom look.
Six years ago, Timothy Spahr and Carl Hergenrother, then UA undergraduate
students, with faculty sponsor Stephen M. Larson of the UA Lunar and
Planetary Laboratory, launched the Bigelow Sky Survey at the university's
16-inch Schmidt telescope near Mount Bigelow, Ariz. It has been a part time
photographic survey, inspired by and patterned after the systematic
photographic search conducted at the Palomar Observatory Schmidt telescope
in southern California by the late Eugene Shoemaker of the U.S.Geological
Survey and his wife, Carolyn.
With the photographic system, observers took two images, 30 minutes apart,
of the exact same region of the sky, then compared the images under a
stereo microscope. Moving objects appeared to "float" above the flat star
background. It took 60 minutes to thoroughly scan a pair of films, Larson
Program observers Spahr and Hergenrother made the Bigelow survey's most
famous discovery, a near-Earth asteroid 200 meters in diameter, or roughly
four times the size of the impactor that produced Meteor Crater, Ariz.,
that missed Earth by about 280,000 miles on May 19, 1996.
Several months ago, Larson's group started a major NASA-funded upgrade of
the system and renamed the project the Catalina Sky Survey.
Ultimately, Larson said, "With current and anticipated improvements, we will
be able to detect an object in one-fortieth the exposure time needed for
photographic observations, or we will be able to detect objects 14 times
fainter while covering the same area as film with our new system. And this
does not include improvements using software to replace eyeball scanning."
The Catalina Sky Survey is unusual in that program observers hunt for Earth-
orbit crossing asteroids and comets above the plane of the ecliptic, or the
plane in which the planets revolve.
Other surveys, including the productive and pioneering UA Spacewatch,
directed by planetary sciences Professor Tom Gehrels, hunt for near-Earth
asteroids along the ecliptic. Most Earth-crossing asteroids can be found when
their slightly inclined orbits cross the ecliptic plane twice each orbit so
long as they are near enough or bright enough and are observed for a long
enough time. Sometimes, they are close enough to the Earth that perspective
causes them to appear outside of the ecliptic.
"Away from the ecliptic plane, we don't see as many of these objects because
there are fewer of them," Larson said. "But while we don't find as many
objects, a higher percentage of those we do find have interesting orbits. We
aren't able to detect objects as faint and small as the larger (36-inch)
Spacewatch telescope, but we will cover much more area for the brighter and
potentially more dangerous asteroids."
Larson now is collecting data to test software that operates an electronic
camera on the newly computer-controlled UA Catalina Schmidt telescope. The
heart of the camera is a very large, very sensitive 4,096 by 4,096-pixel
charge-coupled device (CCD). It is the same type of chip used in Eleanor
Helin's Near-Earth Asteroid Tracking program, sponsored jointly by NASA, the
NASA Jet Propulsion Lab and the U.S. Air Force. The device can record light
50 times fainter than can be captured by the most sensitive photographic
The Catalina telescope now electronically images a 3x3-degree field of view,
or a square patch of sky equivalent to six lunar diameters on a side. It is
capable of finding objects as faint as 20th magnitude, which is approaching
the sky background level generated by scattered city light and auroral
airglow that brightens Earth's upper atmosphere.
The astronomers take from three to five electronic images of the exact same
sky region about 30 minutes apart. When they perfect the performance of the
telescope, they will be able to precisely align the multiple electronic
images so that stars and galaxies appear as single stationary sources of
light and moving objects are seen to move across the screen.
Other proposed improvements to the Catalina Sky Survey include increasing the
telescope's light gathering power with a larger entrance corrector plate, and
to increase computer capacity. Each electronic image is 32 megabytes and
requires near real-time processing to extract moving objects."We now have
this fairly fast computer, but it has to cope with 16 million pixels per
image," Larson said.
John Brownlee, a recent graduate of the UA geosciences program, is programming
the computer to coordinate the telescope and CCD control with data reduction
and object detection. Automation of repetitive tasks and sequence flexibility
to compensate for cloudy weather will be major goals.
Spahr recently finished his dissertation, which included data from the Bigelow
Sky Survey, and has been awarded his doctorate from the University of Florida.
Graduate school has kept him away from Tucson during most of the upgrade, but
he soon will rejoin the survey effort he initiated. This time, he will have
state-of-the-art survey tools.
The Catalina Schmidt telescope is available to the sky survey team most of the
time, so only the bright moon limits observing time, Larson added. "With enough
money to hire people, which is the big operational expense, we could observe 21
nights a month."
Ideally, the several near-Earth asteroid surveys could cover the sky two-to-
three times a month, he said. "The Catalina Sky Survey team anticipates making
significant contributions to the NASA near-Earth asteroid inventory effort."