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

From: Sterling K. Webb <kelly_at_meteoritecentral.com>
Date: Thu Apr 22 09:50:29 2004
Message-ID: <3CC64464.20D72FD7_at_bhil.com>

Hi, Ron, List,

    I sense in this news of so many binary KBO's a real paradox
in dynamics. If binary KBO's are formed by a close encounter or
collision with a heavier object (like the recent news of so many
binary NEA's), that argues for a high frequency of close
encounters (to form so many), but the persistence of a binary
like 1998 WW31 with a very eccentric companion argues that
encounters must be very rare, since such a secondary object would
be easily stripped away by an encounter. Obviously, we can't have
it both ways!
    And if binary KBO's are formed not by fissioning during a
close encounter with a heavier object, then they must have formed
by two objects encountering each other and being captured into a
mutual orbit, but... The same paradox applies to this case, too.
    The reasons for thinking encounters would be rare is simple
geometry. The volume enclosed by the Kuiper Belt (40 AU to 80 AU)
is, well, astronomical! I tried to compute it in cubic kilometers
and my calculator ran out of exponents. If there were enough
encounters to form a reasonable percentage of binary KBO's, even
over a 4 billion year time span, their number would have to be
immense, vastly greater than what we now believe their population
to be.
    There has to be a missing piece in this puzzle...


Sterling K. Webb
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Ron Baalke wrote:

> 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 Wed 24 Apr 2002 01:36:37 AM PDT