[meteorite-list] Kissing in the Kuiper Belt

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Sun Nov 28 23:49:13 2004
Message-ID: <200411290449.UAA20483_at_zagami.jpl.nasa.gov>

http://skyandtelescope.com/news/article_1394_1.asp

Kissing in the Kuiper Belt
By Robert Naeye
Sky & Telescope
November 23, 2004

Among the strangest denizens of the solar system are
contact binaries. In these systems, two minor planets orbit each other
so closely that they literally or nearly touch end-to-end - resulting in
a peanut-like overall shape. Until now, astronomers had found only two
possible contact binaries of relatively large size: the main-belt
asteroid 216 Kleopatra and the Trojan asteroid 624 Hektor. Now Scott S.
Sheppard (Carnegie Institution of Washington) and David C. Jewitt
(University of Hawaii) may have found a third example: a Kuiper-Belt
object orbiting beyond Pluto.

The object, 2001 QG298, orbits so far from the Sun that even the Hubble
Space Telescope has no chance of resolving a peanut shape. But after
measuring the object's changing brightness in 2002 and 2003 with the
University of Hawaii's 2.2-meter telescope and the 10-meter Keck I
telescope, Sheppard and Jewitt noticed something unusual. Its brightness
varies by a whopping 1.14 magnitude every 6.89 hours. Yet the object's
colors do not change, which suggests that dark spots rotating in and out
of view are not causing the brightness changes.

Only three other solar-system objects larger than 50 kilometers across
range in brightness by more than 1 magnitude. Two of them are Kleopatra
and Hektor; the third is Saturn's peculiar moon Iapetus, which displays
a very dark leading hemisphere and a very bright trailing hemisphere.
But Iapetus represents an unusual case because it is locked in
synchronous rotation with Saturn, so its leading edge may be sweeping up
dark material kicked off from Saturn's outer satellites.

With an average diameter of about 180 kilometers, 2001 QG298 is large
enough that it should be nearly spherical. But the object is not
spinning fast enough for rotation to whirl it into an elongated shape.
The simplest explanation for the brightness variations is that two
roughly spherical and equal-sized bodies eclipse each other periodically
every 6.89 hours, which means they must be very close together. We view
them along their equators, which maximizes the eclipsing effect.

"We believe 2001 QG298 is a contact binary," said Sheppard, as he
reported the team's findings at the November meeting of the American
Astronomical Society's Division of Planetary Sciences in Louisville,
Kentucky.

Sheppard and Jewitt have found other possible contact-binary Kuiper Belt
objects (KBOs). Given the number of KBOs they have observed, and the
fact that other KBO contact binaries might be viewed pole-on (which
makes them harder to categorize), Sheppard and Jewitt estimate that at
least 10 to 20 percent of all large KBOs might be contact binaries with
similarly-sized components. "The number of contact binaries with one
component much larger than the other is probably much higher," says
Sheppard, "but these don't make as large brightness variations and thus
are not as easy to distinguish."

These close pairs probably formed early in the solar system's history
when two bodies approached each other and went into mutual orbit after
exchanging orbital energy with other bodies nearby.
Received on Sun 28 Nov 2004 11:49:03 PM PST