[meteorite-list] Case of Sedna's Missing Moon Solved

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue Apr 5 21:15:38 2005
Message-ID: <200504060115.j361FDI01565_at_zagami.jpl.nasa.gov>

http://cfa-www.harvard.edu/press/pr0510.html

Harvard-Smithsonian Center for Astrophysics
Release No.: 05-10
For Release: April 5, 2005

High-resolution artwork to accompany this release is posted online at:
http://www.cfa.harvard.edu/press/pr0510image.html
    
Case of Sedna's Missing Moon Solved

Cambridge, MA--When the distant planetoid Sedna was discovered on the
outer edges of our solar system, it posed a puzzle to scientists. Sedna
appeared to be spinning very slowly compared to most solar system
objects, completing one rotation every 20 days. Astronomers hypothesized
that this world possessed an unseen moon whose gravity was slowing
Sedna's spin. Yet Hubble Space Telescope images showed no sign of a moon
large enough to affect Sedna.

New measurements by Scott Gaudi, Krzysztof (Kris) Stanek and colleagues
at the Harvard-Smithsonian Center for Astrophysics (CfA) have cleared up
this mystery by showing that a moon wasn't needed after all. Sedna is
rotating much more rapidly than originally believed, spinning once on
its axis every 10 hours. This shorter rotation period is typical of
planetoids in our solar system, requiring no external influences to
explain.

"We've solved the case of Sedna's missing moon. The moon didn't vanish
because it was never there to begin with," said Gaudi.

Sedna is an odd world whose extreme orbit takes it more than 45 billion
miles from the Sun, or more than 500 astronomical units (where one
astronomical unit is the average Earth-Sun distance of 93 million
miles). Sedna never approaches the Sun any closer than 80 astronomical
units, and takes 10,000 years to complete one orbit. In comparison,
Pluto's 248-year-long oval orbit takes it between 30 and 50 astronomical
units from the Sun.

"Up until now, Sedna appeared strange in every way it had been studied.
Every property of Sedna that we'd been able to measure was atypical,"
said Gaudi. "We've shown that Sedna's rotation period, at least, is
entirely normal."

Sedna appears unusual in other ways besides its orbit. First and
foremost, it is one of the largest known "minor planets," with an
estimated size of 1,000 miles compared to Pluto's 1,400 miles. Sedna
also displays an unusually red color that is still unexplained.

Initial measurements indicated that Sedna's rotation period was also
extreme - extremely long compared to other solar system residents. By
measuring small brightness fluctuations, scientists estimated that Sedna
rotated once every 20-40 days. Such slow rotation likely would require
the presence of a nearby large moon whose gravity could apply the brakes
and slow Sedna's spin. As a result of this interpretation, artist's
concepts released when Sedna's discovery was announced showed a
companion moon. One month later, images taken by NASA's Hubble Space
Telescope demonstrated that no large moon existed.

In true detective fashion, Gaudi and his colleagues re-investigated the
matter by observing Sedna using the new MegaCam instrument on the
6.5-meter-diameter MMT Telescope at Mount Hopkins, Ariz. They measured
Sedna's brightness looking for telltale, periodic brightening and
dimming that would show how fast Sedna rotates.

As noted by Matthew Holman, one of the members of the CfA team, "The
variation in Sedna's brightness is quite small and could have been
easily overlooked."

Their data fits a computer model in which Sedna rotates once every 10
hours or so. The team's measurements definitively rule out a rotation
period shorter than 5 hours or longer than 10 days.

While these data solve one mystery of Sedna, other mysteries remain.
Chief among them is the question of how Sedna arrived in its highly
elliptical, eons-long orbit.

"Theorists are working hard to try to figure out where Sedna came from,"
said Gaudi.

Astronomers will continue to study this strange world for some time to
come.

"This is a completely unique object in our solar system, so anything we
can learn about it will be helpful in understanding its origin," said
Stanek.

This research has been submitted to The Astrophysical Journal Letters
for publication and is posted online at

http://arxiv.org/abs/astro-ph/0503673.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for
Astrophysics (CfA) is a joint collaboration between the Smithsonian
Astrophysical Observatory and the Harvard College Observatory. CfA
scientists, organized into six research divisions, study the origin,
evolution and ultimate fate of the universe.

For more information, contact:

David Aguilar, Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
Phone: 617-495-7462 Fax: 617-495-7468
daguilar_at_cfa.harvard.edu

Christine Pulliam
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
Phone: 617-495-7463, Fax: 617-495-7016
cpulliam_at_cfa.harvard.edu
Received on Tue 05 Apr 2005 09:15:13 PM PDT