[meteorite-list] If The Horseshoe Fits? (Asteroid 2002 AA29)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:53:24 2004
Message-ID: <200212021638.IAA28379_at_zagami.jpl.nasa.gov>

http://www.astronomy.com/Content/Dynamic/Articles/000/000/001/118nkdjz.asp

If the Horseshoe Fits?

An unusual asteroid with an Earthlike, "horseshoe" orbit periodically
becomes a quasi-satellite of Earth.

by Phil Plait and Vanessa Thomas
astronomy.com
December 2, 2002

A near-Earth asteroid discovered in January is one of the most peculiar
objects in the solar system. Its Earthlike orbit and the gravitational
influence of the Earth and sun make it a natural companion to our planet,
though it isn't a satellite. From our point of view, this odd object loops
around our planet's orbit path and appears to change direction each time
it approaches our world.

The 100-meter-wide asteroid was found by the Lincoln Near Earth Asteroid
Research (LINEAR) survey, which automatically sweeps the sky looking for
asteroids that venture close to Earth. Designated 2002 AA29, the object's
average distance from the sun is 0.999996 times that of Earth, or just a few
hundred miles different than Earth's average distance. The asteroid's orbit
is also very circular, even more so than Earth's. The main difference is
that it orbits the sun at a fair tilt: its orbital plane is tipped by 10.7
degrees relative to Earth's. Because of this inclination, it can only get
about 3 to 4 million miles from our planet at its closest approach, and
therefore poses no impact threat.

Like Earth, the asteroid orbits the sun once per year. However, it
moves at different speeds throughout the year. During part of its
orbit, it is closer to the sun than Earth is and travels a bit faster than
Earth does. After it gains on us, our planet's gravity lends energy to
the object, which moves it into a higher orbit slightly farther from the
sun than we are. Due to the peculiar nature of orbital mechanics, this
causes the asteroid to slow a bit, and Earth moves ahead. When it does,
Earth's gravity takes away orbital energy from the asteroid. The object
then moves in a bit closer to the sun and speeds up. Six months later it
catches up and the dance is played out once again.

Each of these "loops" takes one year to complete. Because of the slight
differences in their orbits, Earth and 2002 AA29 gradually drift in
relation to one another over the years - first farther apart and then
closer together. Every 95 years, the asteroid approaches Earth from a
different direction, and then appears to start moving away again in the
opposite direction. If 2002 AA29's path around the sun is plotted such
that Earth is kept fixed, the object traces a horseshoe shape during its
orbit, with the tips of the horseshoe on opposite sides of Earth.

The next close approach by 2002 AA29 will be on January
8, 2003, when it will be about 3.6 million miles away and
will shine at a meager 19th magnitude. It will approach
Earth again in 2098, after it has completed another
95-year horseshoe.

Another object with a horseshoe orbit around Earth is
known; named Cruithne, it was discovered in 1986.
However, its orbit is more elliptical than 2002 AA29's,
and thus is not as close a companion.

This asteroid is unique in yet another way. Because its
orbit is not exactly like Earth's, the nature of the
encounters between the two changes cyclically over the
course of thousands of years. For a relatively short time
during this period, 2002 AA29's orbit brings it close
enough so that it becomes a quasi-satellite of Earth. Over
50 years or so it remains close by Earth, appearing to orbit
our planet slowly once per year (although actually still
orbiting the sun), and then eventually pulls back away. It
is the only known object to do this. Astronomers believe
this last happened between 550 and 600 A.D. They predict
it will happen again around the year 2600.

To learn more about this asteroid and horseshoe orbits, visit the
discoverers' 2002 AA29 website:

http://www.astro.queensu.ca/~wiegert/AA29/AA29.html
Received on Mon 02 Dec 2002 11:38:04 AM PST