[meteorite-list] More Asteroids Caught In Trojan Force Traps

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:37:34 2004
Message-ID: <200012121705.JAA04210_at_zagami.jpl.nasa.gov>

http://helix.nature.com/nsu/001214/001214-6.html

More asteroids caught in Trojan force traps
PHILIP BALL
Nature Science Update
December 12, 2000

The asteroid belt between Mars and Jupiter may not be alone in harbouring
debris left over from the formation of the planets. New calculations hint
there could be similar asteroid hoards associated with planets nearer the
Sun. This lends added urgency to scientists' eager scrutiny of the belt once
regarded as the trash heap of the solar system, now deemed a potential
source of rare minerals or of bodies on a collision course with Earth.

Wyn Evans and Serge Tabachnik of the University of Oxford, UK, have worked
out that there are probably other asteroid pockets in the inner Solar
System, which are being shepherded by Mars, Earth, Venus and Mercury. And
the duo suggests where, when and how astrophysicists should look for them.

The asteroids associated with the giant planet Jupiter are known as 'the
Trojans'. They owe their existence to a peculiarity of gravitational
interactions. Isaac Newton showed in the seventeenth century how gravity
holds a planet in orbit around the Sun. But the equations to describe the
motion of three mutually gravitating bodies, rather than just two, are
fiendishly difficult to solve.

In the eighteenth century, the French mathematician Joseph Louis Lagrange
simplified things. He showed that if one of the bodies orbits around
another, like a planet around the Sun, a third small body can get trapped at
any of five specific points -- Lagrange points -- relative to the orbiting
planet.

In 1906, astrophysicists detected the first of Jupiter's Trojan asteroids at
one of its Lagrange points. Now, 470 Trojans are known and there may be as
many as 2,500, some with diameters that could exceed 15 kilometres.

Saturn too has some small moons captured at its Lagrange points. And in
1990, a Trojan-like asteroid called '5261 Eureka' was discovered on Mars'
orbit, showing that you don't have to be a giant planet to secure your own
asteroids.

But in the Monthly Notices of the Royal Astronomical Society1,2, Evans and
Tabachnik ask: how many asteroids are associated with the inner planets, and
how easy would it be to spot them, given that they'd be proportionately
smaller than those of Jupiter?

These questions can only be answered through computer simulations of the
planetary motions. This involves scattering 'candidate Trojans' around the
orbits of the inner planets and then seeing whether the simulations trap
them at Lagrange points or send them hurtling off elsewhere.

The confidence in the results depends on sampling enough possibilities and
on running the simulations for long enough compared with the age of the
Solar System. Evans and Tabachnik ran their simulations to cover up to 100
million years -- ten times longer than most previous studies, but still
quite short relative to the Solar System's 4.6 billion-year age.

The simulations reveal that, like Mars, Venus and Earth can shepherd Trojans
at their Lagrange points. For Mercury, very few candidate Trojans survive in
the traps because the planet is small and has a rather unusual orbit.

Astrophysicists have already scanned the Lagrange points of Mars and Earth
and found nothing, aside from Eureka and one other martian Trojan. But Evans
and Tabachnik have used their results to propose optimal search strategies
that could be conducted with existing telescopes and might prove more
fruitful.

                   --------------------------------------

  1. Tabachnik, S. A. & Evans, N. W. Asteroids in the inner solar system -
     I. Existence. Monthly Notices of the Royal Astronomical Society 319,
     63-79 (2000).
  2. Evans, N. W. & Tabachnik, S. A. Asteroids in the inner solar system -
     II. Observable properties. Monthly Notices of the Royal Astronomical
     Society 319, 80-94 (2000).
Received on Tue 12 Dec 2000 12:05:51 PM PST