[meteorite-list] Earth-Companion Asteroid Discovered in Horseshoe-Shaped Orbit (2010 SO16)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 7 Apr 2011 09:16:50 -0700 (PDT)
Message-ID: <201104071616.p37GGoB9018796_at_zagami.jpl.nasa.gov>

http://www.technologyreview.com/blog/arxiv/26608/

Earth-Companion Asteroid Discovered in Horseshoe-Shaped Orbit

Earth shares its orbit around the Sun with an asteroid in an exotic
horseshoe-shaped orbit, say astronomers

Technology Review
April 5, 2011

In the 1969 film Doppelganger, scientists discover and then visit an
Earth-like planet sharing our orbit but on exactly the other side of the
Sun.

Since then, astronomers have ruled out the possibility of such a planet
on the grounds that its gravitational effects on other planets and
spacecraft would be easy to see.

But that doesn't rule out the possibility of smaller objects sharing
Earth's orbit and today, Apostolos Christou and David Asher at the
Armagh Observatory in Northern Ireland say they've found one--an
asteroid called 2010 SO16

Near-Earth asteroids are common but SO16 is in a category of its own.
First and foremost, it has an exotic horseshoe-shaped orbit (see diagram
above) which astronomers believe to be very rare.

Its worth taking a few moments to think about horseshoe orbits. Two
points are worth bearing in mind. First, objects further from the Sun
than Earth, orbit more slowly. Second, objects that are closer to the
Sun orbit more quickly than Earth.

So imagine an asteroid with an orbit around the Sun that is just a
little bit smaller than Earth's. Because it is orbiting more quickly,
this asteroid will gradually catch up with Earth.

When it approaches Earth, the larger planet's gravity will tend to pull
the asteroid towards it and away from the Sun. This makes the asteroid
orbit more slowly and if the asteroid ends up in a orbit that is
slightly bigger than Earth's, it will orbit the Sun more slowly than
Earth and fall behind.

After that, the Earth will catch up with the slower asteroid in the
bigger orbit, pulling it back into the small faster orbit and process
begins again.

So from the point of view of the Earth, the asteroid has a
horseshoe-shaped orbit, constantly moving towards and away from the
Earth without ever passing it. (However, from the asteroid's point of
view, it orbits the Sun continuously in the same direction, sometimes
more quickly in smaller orbits and sometimes more slowly in bigger orbits.)

For SO16, the period of this effect is about 350 years

Horseshoe orbits are thought to be very unstable, since any small
gravitational tug can destroy the fragile resonance that has been set
up. However, SO16's orbit is surprisingly robust.

Christou and Asher simulated its orbit with slightly different values
for parameters such as its semi-major axis. In these simulations, SO16
remained in a horseshoe-shaped orbit for at least 120,000 years and
sometimes for more than a million years.

Astronomers know of three other horseshoe companions for Earth but these
are all much smaller (SO16 is a few hundred meters across) and none have
orbits that are likely to survive for more than a few thousands years.

That makes SO16 kind of special. For anybody willing and able to look,
it is currently near one of its points of closest approach, with an
absolute magnitude of about 20, lagging the Earth by 0.13 AU, like a
stray puppy.

And it will be there for some time, say Christou and Asher. "It will
remain as an evening object in the sky for several decades to come."

Ref: arxiv.org/abs/1104.0036 <http://arxiv.org/abs/1104.0036> : A
Long-Lived Horseshoe Companion To The Earth
Received on Thu 07 Apr 2011 12:16:50 PM PDT