[meteorite-list] Second Moon Spotted Circling Cigar-Shaped World (2003 EL61)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Dec 1 18:51:16 2005
Message-ID: <200512012349.jB1Nnjd04964_at_zagami.jpl.nasa.gov>

http://www.newscientistspace.com/article/dn8402--second-moon-spotted-circling-cigarshaped-world.html

Second moon spotted circling cigar-shaped world
Maggie McKee
New Scientist
01 December 2005

Astronomers have spotted a second moon around a massive, cigar-shaped
world at the fringes of the solar system. The discovery suggests
multiple moons orbit many large, distant objects - but their unusual
orbits raise questions about just how they could have formed.

The moons orbit a rocky body called 2003 EL61, which is a particularly
bizarre member of the Kuiper Belt - a vast ring of icy objects beyond
Neptune. 2003 EL61 rotates once every four hours - faster than any other
Kuiper Belt Object (KBO) - and is shaped like a cigar that stretches 2000
kilometres on its longest side - nearly the diameter of Pluto.

In January 2005, a team led by Mike Brown at Caltech in Pasadena,
California, US, discovered it is orbited by a moon that may be 300 km
wide. The moon takes about 49 days to orbit 2003 EL61 at a distance of
49,000 km.

Now, the same team has spotted another, fainter, moon in three of five
earlier images taken using the Keck Observatory in Hawaii. The new moon
may be just 150 km across and appears to orbit once every 34 days at a
distance of 39,000 km. But the new moon travels in a plane that is
tilted by about 40? to the orbital plane of the bigger, brighter moon.
          
Hot crash

The researchers believe the moons coalesced from debris after another
large KBO smashed into 2003 EL61 when the solar system was forming, more
than 4 billion years ago. In their scenario, the crash heated 2003 EL61
so much that it lost most of its ice and was left as mostly rock. It
also set the hot rock spinning so fast it got stretched into its cigar
shape before cooling.

This explains most of the system's features, says Brown. But he says the
"tough question" is why the two tiny moons are in such different orbital
planes. "My best guess is that when both moons were closer, they
gravitationally interacted and moved," he told New Scientist.

Robin Canup, a planetary scientist at the Southwest Research Institute
in Boulder, Colorado, US, agrees that a collision could explain the
presence of the moons and the larger object's spin. But she says the
difference in the moons' orbits is "surprising in this context, and may
point to a more complex history for this system".

Mass means moons

Pluto recently became the first KBO known to boast more than one moon.
Now, the discovery of a second multiple system "suggests there might be
more out there," says Brown.

Brown adds that three out of four of the largest KBOs - Pluto, 2003 EL61
and 2003 UB313 (currently known as the "tenth planet") - all have at
least one moon. That is a significantly higher proportion than the 10%
of smaller KBOs seen to have moons, suggesting that mass is an important
factor in forming and keeping satellites.

"My guess is that while many objects underwent collisions, only the most
massive ones retained a disc of material out of which moons could
coalesce," he says.
Received on Thu 01 Dec 2005 06:49:44 PM PST