[meteorite-list] Sedna Mystery Deepens With Hubble Images of Farthest Planetoid

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:31:14 2004
Message-ID: <200404141649.JAA03742_at_zagami.jpl.nasa.gov>

Donald Savage
Headquarters, Washington April 14, 2004
(Phone: 202/358-1547)

Ray Villard
Space Telescope Science Institute, Baltimore
(Phone: 410/338-4514)

Robert Tindol
California Institute of Technology, Pasadena, Calif.
(Phone: 626/395-3631)

RELEASE: 04-125

SEDNA MYSTERY DEEPENS WITH HUBBLE IMAGES OF FARTHEST
PLANETOID

     Astronomers studying 35 NASA Hubble Space Telescope
(HST) images of the solar system's farthest known object,
unofficially named Sedna, are surprised the object does not
appear to have a companion moon of any substantial size.

This unexpected result might offer new clues to the origin
and evolution of objects on the far edge of the solar system.

Sedna's existence was announced on March 15. Its discoverer,
Mike Brown of the California Institute of Technology,
Pasadena, Calif., was so convinced it had a satellite, that
an artist's concept of Sedna released to the media included a
hypothetical moon.

Brown's prediction was based on the fact, Sedna appears to
have a very slow rotation that could best be explained by the
gravitational tug of a companion object. Almost all other
solitary bodies in the solar system complete a spin in a
matter of hours.

"I'm completely baffled at the absence of a moon," Brown
said. "This is outside the realm of expectation and makes
Sedna even more interesting. But I simply don't know what it
means." Immediately following the announcement of the
discovery of Sedna, NASA astronomers turned the HST toward
the new planetoid to search for the expected companion moon.
The space-based platform provides the resolving power needed
to make such precision measurements in visible light.
"Sedna's image isn't stable enough in ground-based
telescopes," Brown said.

Surprisingly, the HST images, taken March 16 with the new
Advanced Camera for Surveys, only show the single object
Sedna, along with a faint, very distant background star in
the same field of view.

Even with Hubble's crisp view, it may just be barely
resolving the disk of mysterious Sedna, Brown said. It's
equivalent to trying to see a soccer ball 900 miles away.
This would place an upper limit in the object's size of being
approximately three-quarters the size of Pluto, or about
1,000 miles across.

But Brown predicted a satellite would pop up as a companion
"dot" in Hubble's precise view. But the object is not there.
There is a very small chance, it might have been behind Sedna
or transiting in front of it, so it could not be seen
separately from Sedna in the HST images.

Brown based this prediction on his earlier observations of
apparent periodic changes in light reflecting from Sedna's
mottled surface. The resulting light curve gives a rotation
period of 40 days. If true, Sedna would be the slowest
rotating object in the solar system after Mercury and Venus,
whose slow rotation rates are due to the tidal influence of
the sun.

One easy way out of this dilemma is the possibility the
rotation period is not as slow as astronomers thought. But
even with a careful reanalysis, the team remains convinced
the period is correct. Brown admits, "I'm completely lost for
an explanation as to why the object rotates so slowly."

Small bodies like asteroids and comets typically complete one
rotation in a matter of hours. Pluto has a six-day period
from being tidally locked to the revolution of its satellite
Charon. The HST was the first telescope to resolve Pluto and
Charon as two separate bodies. NASA's forthcoming James Webb
Space Telescope will provide a platform for further high-
resolution studies of infrared light from such distant, cold
bodies in our solar system.

Electronic images of Sedna and additional information are
available on the Internet, at:

http://hubblesite.org/news/2004/14


-end-
Received on Wed 14 Apr 2004 12:49:43 PM PDT