[meteorite-list] Asteroids Near Jupiter are Really Comets
From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed Feb 1 16:00:11 2006
Asteroids Near Jupiter are Really Comets
By Ker Than
01 February 2006
Two objects lurking near Jupiter and once considered rocky asteroids
have turned out to be comets made up mostly of ice and dirt.
Using the Keck II Laser Telescope in Hawaii, astronomers found that the
two objects, 617 Patroclus and its companion, Menoetius, had a density
of only 0.8 grams per cubic centimeters - only a third that of rock.
Most likely, the researchers say, Patroclus and Menoetius are comets,
which are typically composed mainly of water ice and therefore much less
dense than asteroids.
The finding could mean that many or most of asteroid-like objects
hovering around Jupiter and known as Trojans are actually comets that
originated much farther from the Sun and which were captured by the
giant gas planet when the solar system was still young.
The findings were detailed in the Feb. 2 issue of the journal Nature.
Patroclus and Menoetius are the only known binary objects around
Jupiter. The pair orbit around each other while floating 465 million
miles (750 million kilometers) from Jupiter in one of gas planet's two
so-called Lagrange points. At these points, the gravitational field of
Jupiter and the Sun are perfectly balanced, and objects can be captured
and brought to relative rest. Jupiter has two Lagrange points, one in
front and the other behind as the planet orbits the Sun.
Patroclus and Menoetius are estimated to be about 76 miles (122
kilometers) and 70 miles (112 kilometers) wide, respectively. The two
objects are not the first to be mistaken for asteroids: in 1999,
astronomers determined that C/199 J3 was also a comet.
Because most comets are thought to form in the Kuiper Belt, a distant
region of the solar system outside the orbit of Neptune, the researchers
think Patroclus and Menoetius formed about 650 million years after the
formation of the solar system.
"It's our suspicion that the Trojans are small Kuiper Belt objects,"
said study leader Franck Marchis, an astronomer at the University of
According to one hypothesis proposed by the researchers, Jupiter
captured the comets at a time when the large gas planets were orbiting
much closer to the Sun.
During this early period in the solar system, the gas planets were
enveloped by billions of large asteroids called planetesimals. It's
thought that interactions with planetesimals caused the large gas
planets to migrate outwards to their present positions. As the planets
migrated, the swarming planetesimals were tossed around like confetti.
The majority of them would have been hurled into the outer reaches of
the solar system to form the Kuiper Belt, while a smaller number would
have been captured in the Lagrange points of Jupiter and the other gas
Received on Wed 01 Feb 2006 03:58:24 PM PST