[meteorite-list] Abundance of Cometlike Objects With Moons Stuns Experts

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:50:29 2004
Message-ID: <200204232004.NAA21047_at_zagami.jpl.nasa.gov>

Abundance of Cometlike Objects With Moons Stuns Experts
By KENNETH CHANG
New York Times
April 23, 2002

Even small cometlike bodies at the edge of the solar system
often have companion moons, to the surprise of astronomers
who cannot yet explain how such tenuous gravitational
pairings formed.

Writing in the current issue of the journal Nature, a team
of American and French astronomers describe the looping
elliptical orbits of 1998 WW31, a small icy clump 4.3
billion miles from the Sun, and its moon.

The pair is part of the Kuiper Belt, a ring of debris
beyond the orbit of Neptune. When a Kuiper Belt object is
nudged by a passing object's gravity and falls into the
inner solar system, it becomes a comet.

Discovered four years ago, 1998 WW31 is one of more than
500 bodies that have been cataloged in the Kuiper Belt. In
follow-up observations in 2000, astronomers led by Dr.
Christian Veillet of the Canada-France-Hawaii Telescope in
Kamuela, Hawaii, noticed that 1998 WW31 appeared elongated,
like a blurry peanut. Comparing their images with those
taken earlier, they found that the shape of the peanut
changed over time, suggesting the motion of a moon around
1998 WW31.

The astronomers announced the discovery last year, the
first known around a Kuiper Belt object, unless one counts
Pluto's moon Charon. (Some regard Pluto as the largest of
the Kuiper Belt objects.)

With help from new photographs by the Hubble Space
Telescope, the same team of astronomers has now mapped out
the trajectories. Roughly the same size - 1998 WW31 is
estimated at 75 to 90 miles wide; its moon is 60 to 75
miles wide - the two twirl around each other in a slow,
highly elliptical dance. At their closest, they pass 2,500
miles from each other. At their most distant, they are
25,000 miles apart. They take 570 days to revolve around
each other.

"Their orbital motion is very, very eccentric," Dr. Veillet
said.

Astronomers once thought the gravitational pull of small
bodies like asteroids and comets to be too slight to hold
onto moons. In 1994, they were astonished when photographs
from NASA's Galileo spacecraft revealed a tiny moon
circling the asteroid Ida. A handful more moons have been
discovered since among the thousands of asteroids between
the orbits of Mars and Jupiter.

Moons around Kuiper Belt objects may be unexpectedly
common. In the past year, astronomers have found moons
about six more Kuiper Belt objects, and they have not yet
examined most of them.

"That's the extraordinary thing about this," said Dr. David
Jewitt, a professor of astronomy at the University of
Hawaii and one of the scientists who found the first Kuiper
Belt object in 1992. No one predicted the observed
abundance of moons. "It just happened," he said.

Low-speed collisions between two Kuiper Belt objects may
dissipate enough energy to allow the two to go into orbit
around each other. A collision could also split one of the
objects into a pair. "Then the details after that are
hazy," Dr. Jewitt said.

Now, most Kuiper Belt objects are too small, dim and
distant for astronomers to learn much about them. They
could get precise measurements of the sizes of 1998 WW31
and its moon when their orbits turn edge-on toward Earth,
and one passes directly in front of the other.

Astronomers have plenty of time to get ready, but many will
not be alive to observe it. The earliest the eclipses may
occur is around 2050.

"I think I'm not likely to be," said Dr. Veillet, 54.
Received on Tue 23 Apr 2002 04:04:32 PM PDT