[meteorite-list] New Trojan Asteroid Hints at Huge Neptunian Cloud

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri Jun 16 12:17:52 2006
Message-ID: <200606161609.JAA00865_at_zagami.jpl.nasa.gov>

http://www.newscientistspace.com/article/dn9340-new-trojan-asteroid-hints-at-huge-neptunian-cloud.html

New Trojan asteroid hints at huge Neptunian cloud
Kelly Young
New Scientist
15 June 2006
 
A newly discovered asteroid in Neptune's orbit indicates the existence
of a much larger, but as-yet-unseen, cloud of rocks in that region. The
asteroids in Neptune's orbit might even outnumber those in the main
asteroid belt between Mars and Jupiter, the new research suggests.

The asteroid was discovered by Scott Sheppard of the Carnegie
Institution of Washington, and Chadwick Trujillo of the Gemini
Observatory in Hawaii, both in the US. They used Carnegie's
Magellan-Baade 6.5-metre telescope in Chile and the 8.2-metre Gemini
Telescope.

It is one of four known Neptunian "Trojan" asteroids, which orbit in
lockstep with the planet. Two of the Trojans were discovered by Sheppard
and Trujillo in 2004 and 2005.

The asteroids orbit 60?, or about 5 billion kilometres, ahead of Neptune
on its circular orbit around the Sun, which is a gravitationally stable
location called a Lagrange point. But the newly-found asteroid is unique
in that its orbit is tilted 25? relative to the plane of the solar system.

Asteroids with such tilts are only rarely in the particular regions
surveyed by the astronomers. So such a sighting means observers can make
a statistical inference that there must be many more. "This finding
strongly supports the idea that Neptune Trojans fill a thick disc with a
population comparable to, or even larger than, that of [the 2000 known]
Jupiter Trojans," says Francesco Marzari, at the University of Padova,
Italy, in a commentary on the paper in the journal Science.

Trojan zone

How planets acquire their Trojans has been much discussed by researchers
but the high tilt in this case favours a theory called "freeze-in"
capture, which is thought to have also allowed Jupiter to acquire its
asteroids.

During the evolution of the gas planets, their orbits were erratic and
small bodies in the solar system moved about chaotically. But as the
planets' orbits stabilised, the strength of gravitationally stable
regions in front of and behind the planet's orbit grew.

"At that moment, whatever objects were at the right place at the right
time - in the Trojan zone - were frozen," says Eugene Chiang, at the
University of California Berkeley, who discovered the first Neptune Trojan.

Another unique feature is that all four Neptune Trojans appear
relatively red, suggesting they all have a similar origin. The tint is
similar to Jupiter Trojans, irregular satellites and maybe even comets,
but is distinct from some of the farther-flung Kuiper Belt Objects.
          
Tens of thousands

The known Neptune Trojans are estimated to measure between 60 and 140
kilometres in diameter. From the statistical analysis based on the 25?
tilt, the researchers infer that Neptune may have between five and 20
times more of these large objects than Jupiter. Jupiter only has one
known Trojan in this upper size range. Based on this difference,
astronomers estimate that Neptune's smaller Trojans also outnumber
Jupiter's.

Other research implies that Jupiter may have about the same number of
asteroids as the main asteroid belt, so astronomers indirectly conclude
that Neptune Trojans may outnumber the tens of thousands of asteroids in
the main asteroid belt.

However, because Neptune is farther away, spotting these rocky objects
is difficult. A cloud of asteroids stalking Neptune from behind, in a
corresponding Lagrange point, has not yet been spotted but is predicted
to exist.

"Everyone believes there should be another cloud," Chiang told New
Scientist. "The reason why people haven't found them there is you're
looking right at the Milky Way in that direction." The large number of
stars in the background makes spotting faint asteroids difficult.
However, planetary motions will make for a better view of any trailing
Trojans in about 30 years.

If astronomers prefer not to wait, it may be possible to send the
Pluto-bound New Horizons mission through the predicted asteroid clumping
(see New Horizons blasts off for Pluto
<http://www.newscientistspace.com/article/dn8612.html>).

Journal reference: Science express (DOI: 10.1126/science.1127173)
Received on Fri 16 Jun 2006 12:09:28 PM PDT