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Earth's Peculiar Neighbor


http://focus.aps.org/v4/st16.html

Earth's Peculiar Neighbors
Physical Review Focus
24 September 1999

An asteroid heading toward Earth makes a good movie plot, but astronomers
haven't yet found any heavenly bodies on such a collision course. There are
plenty of asteroids in our neighborhood of the solar system, however, and a
team of physicists has now discovered several new types of
near-Earth-asteroid orbital motion. In the 27 September PRL they describe
their theory and computer simulations of the orbits of three known
asteroids. They conclude that although we are safe from collisions with such
objects, there may be many asteroids inhabiting these newly discovered
orbits, including undiscovered satellites of Earth.

The orbits are examples of "coorbital motion," where the asteroid is locked
into a gravitational dance with a planet and the Sun, rather than orbiting
the Sun independently. In one simple type of coorbital motion an asteroid
could stay close to Earth's orbital track, always remaining some distance
ahead of Earth as the two bodies orbit the Sun. The asteroid could move
slightly faster than Earth, advancing farther around the sun (from Earth's
perspective) with each year. After many years it could advance all the way
around the Sun to a point not far behind Earth, before slowing down again
and appearing from Earth to turn back the other way.

Asteroid (3753) Cruithne appeared to be in such a "horseshoe" orbit when it
was discovered a few years ago, but its highly eccentric (egg-shaped)
trajectory and steep inclination with the plane of the solar system couldn't
be explained by current theories, says Fathi Namouni of Queen Mary and
Westfield College in London. Namouni and his colleagues developed a theory
for coorbital motion based on the long-term average position of the asteroid
as the shape and orientation of its elliptical orbit evolves with time. They
found several new classes of motion, and unlike previous theories, their
predictions are valid for highly eccentric and inclined orbits. To check
their predictions, the team ran computer simulations for a few real
asteroids to see their orbital tracks 105 years into the past and future.
These asteroids exhibited all of the newly discovered behaviors.

In "retrograde satellite motion"--one of the new classes of motion--an
asteroid can slowly orbit a planet at a great distance, perhaps half the
distance between the planet and the sun. Cruithne appears to be in a
"compound" orbit--a combination of retrograde satellite motion and a
modified horseshoe. The team also discovered that high eccentricity
asteroids can be temporarily captured by a planet and remain in stable
coorbital motion for thousands of years before wandering away: Another
asteroid they investigated orbited Earth for 35,000 years before leaving,
according to their computations.

Namouni explains that all of the inner planets are protected from collisions
with asteroids locked in coorbital motion because the orbits are stable,
rather than chaotic. He expects that more asteroids will soon be discovered
inhabiting some of these orbital modes, perhaps even orbiting the Earth in a
region where no one has yet looked.

The theory "may prove to be important in understanding how planets were
formed," says Scott Tremaine of Princeton University, and it may also allow
space mission planners to come up with new gravitational tricks for their
space probes. But he says the main significance of the work is that it
provides a complete classification of coorbital motions, which should allow
a good understanding of other asteroids, including their likelihood of
hitting Earth.

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