[meteorite-list] Neptune May Have Thousands of Escorts

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 30 Jan 2007 11:46:59 -0800 (PST)
Message-ID: <200701301946.LAA20180_at_zagami.jpl.nasa.gov>

http://www.space.com/scienceastronomy/070130_st_neptune_trojans.html

Neptune May Have Thousands of Escorts
By David Powell
space.com
30 January 2007

Neptune may be escorted in its orbit by thousands of asteroid-like
objects, perhaps more than exist in the entire asteroid belt.

So far, five of these enigmatic bodies, known as Trojans, have been
found at one of Neptune's Lagrange points. These are places where
the gravity of a planet and that of the Sun interact to create an
area of gravitationally stability.

Jupiter's Lagrange regions are home to legions of Trojans, and around
2,000 cluster at these gravity graves along Jupiter's orbit 60 degrees
ahead and 60 degrees behind the gas giant.

The first Neptune Trojan was discovered in 2001 as part of the NASA
funded Deep Ecliptic Survey at the Lagrange region 60 degrees and
3.1 billion miles (5 billion kilometers) ahead of Neptune.

A further three Neptune Trojans between 37 and 87 miles (60 and 140
kilometers) in diameter and shaded a pale red color have since been
identified by Scott Sheppard of the Carnegie Institution of Washington
and Chadwick Trujillo of the Gemini Observatory in Hawaiiusing the
6.5-meter Magellan telescope in Chile.

Despite their diminutive size and brightness, the Neptune Trojans
quickly betrayed their existence by their distinct motion against
background stars. The most recent Trojan discovered by Sheppard and
Trujillo is moving at an unusual inclination of 25 degrees relative
to the plane of the solar system (the ecliptic).

"The sky we covered searching for Neptune Trojans was all within 1.5
degrees of the ecliptic," Sheppard said. "High inclination objects will
spend the majority of their time off the ecliptic. Thus detecting a high
inclination Trojan in our survey suggests there is a large population of
such objects. In fact, the high inclination objects appear to outnumber
the low inclination objects by a ratio of four to one."

If so, there would be swarms of Trojans accompanying Neptune, perhaps
up to twenty times more than at Jupiter. The sheer number of Trojans
Neptune is thought to harbor reveal that these objects are an
established part of Neptune's entourage, dating back to shortly after
the planet's formation.

"Neptune cannot currently efficiently capture Trojans for long periods
of time," Sheppard said. "Just after the planet formation epoch
Neptune's orbit was likely much more eccentric due to its interactions
with the other planets. Neptune's interactions with the myriad small
bodies around its orbit which included comets, Kuiper Belt objects and
other debris which formed nearby would have slowly circularized
Neptune's orbit."

This process would have trapped many diverse objects at the Neptune
Lagrange points irrespective of their inclination. This diversity is
exciting as in 2014 we may get the opportunity to see a Trojan up close
courtesy of the New Horizons spacecraft currently en-route to Pluto.

"If a Neptune Trojan could be found which the New Horizons spacecraft
could image it would be one of the highlights of the mission," Sheppard
said. "The Neptune Trojans are very faint and thus hard to observe from
our location on Earth. Thus little is known about their surface
properties or composition. The Neptune Trojans may be a unique type of
solar system object of which no other stable reservoir currently exists."

At present this flyby of a Neptune Trojan is far from a certainty due to
the fact that New Horizons will pass 60 degrees behind Neptune through
the trailing Lagrange region known as "L5" where no Trojans have yet
been identified.

"We are attempting to discover possible Neptune L5 Trojans, but because
of the high background star confusion it will be a tough task for the
next several years," Sheppard said. "The constraints on the New Horizons
spacecraft are also severe since it will not be able to maneuver too far
from its current trajectory. Thus, there is only a very low probability
that a Neptune Trojan happens to be in a favorable location for the
spacecraft to encounter."
Received on Tue 30 Jan 2007 02:46:59 PM PST