Centaurs: NASA's WISE Finds Mysterious Centaurs May Be Comets

Centaurs: NASA's WISE Finds
Mysterious Centaurs May Be
Comets
July 25, 2013 — The true identity of
centaurs, the small celestial bodies
orbiting the sun between Jupiter and
Neptune, is one of the enduring
mysteries of astrophysics. Are they
asteroids or comets? A new study of
observations from NASA's Wide-field
Infrared Survey Explorer (WISE) finds
most centaurs are comets.
Until now, astronomers were not
certain whether centaurs are
asteroids flung out from the inner
solar system or comets traveling in
toward the sun from afar. Because of
their dual nature, they take their
name from the creature in Greek
mythology whose head and torso are
human and legs are those of a horse.
"Just like the mythical creatures, the
centaur objects seem to have a
double life," said James Bauer of
NASA's Jet Propulsion Laboratory in
Pasadena, Calif. Bauer is lead author
of a paper published online July 22 in
The Astrophysical Journal . "Our data
point to a cometary origin for most
of the objects, suggesting they are
coming from deeper out in the solar
system."
"Cometary origin" means an object
likely is made from the same
material as a comet, may have been
an active comet in the past, and may
be active again in the future.
The findings come from the largest
infrared survey to date of centaurs
and their more distant cousins, called
scattered disk objects. NEOWISE, the
asteroid-hunting portion of the WISE
mission, gathered infrared images of
52 centaurs and scattered disk
objects. Fifteen of the 52 are new
discoveries. Centaurs and scattered
disk objects orbit in an unstable belt.
Ultimately, gravity from the giant
planets will fling them either closer
to the sun or farther away from their
current locations.
Although astronomers previously
observed some centaurs with dusty
halos, a common feature of
outgassing comets, and NASA's
Spitzer Space Telescope also found
some evidence for comets in the
group, they had not been able to
estimate the numbers of comets and
asteroids.
Infrared data from NEOWISE
provided information on the objects'
albedos, or reflectivity, to help
astronomers sort the population.
NEOWISE can tell whether a centaur
has a matte and dark surface or a
shiny one that reflects more light.
The puzzle pieces fell into place when
astronomers combined the albedo
information with what was already
known about the colors of the
objects. Visible-light observations
have shown centaurs generally to be
either blue-gray or reddish in hue. A
blue-gray object could be an asteroid
or comet. NEOWISE showed that
most of the blue-gray objects are
dark, a telltale sign of comets. A
reddish object is more likely to be an
asteroid.
"Comets have a dark, soot-like
coating on their icy surfaces, making
them darker than most asteroids,"
said the study's co-author, Tommy
Grav of the Planetary Science
Institute in Tucson, Ariz. "Comet
surfaces tend to be more like
charcoal, while asteroids are usually
shinier like the moon."
The results indicate that roughly two-
thirds of the centaur population are
comets, which come from the frigid
outer reaches of our solar system. It
is not clear whether the rest are
asteroids. The centaur bodies have
not lost their mystique entirely, but
future research from NEOWISE may
reveal their secrets further.
JPL, managed by the California
Institute of Technology in Pasadena,
managed and operated WISE for
NASA's Science Mission Directorate.
The NEOWISE portion of the project
was funded by NASA's Near Earth
Object Observation Program. WISE
completed its key mission objective,
two scans of the entire sky, in 2011
and has been hibernating in space
since then.
For more information about the WISE
mission, visit: http://www.nasa.gov/
wise .