Asteroid 4179 Toutatis/1989 AC is due to fly by the Earth early on Wednesday 12 December 2012 at a distance of about 7 million kilometers, which is slightly less than twenty times the distance to the Moon. This is not an unpredicted event; the asteroid passes us every fourth December, which means that a wide array of Earth-based telescopes will be trained upon it. While several objects pass closer to the Earth each year, 4179 Toutatis/1989 AC is quite a large object, roughly 2 km by 5 km, so it may be easier to observe for smaller telescopes.
4179 Toutatis/1989 AC belongs to the Alinda group of asteroids, which have eccentric (highly elliptical) orbits and orbital resonances with both the Earth and Jupiter. An orbital resonance occurs when two bodies periodically encounter one-another in their orbit, and exchange some orbital energy; this effectively means the larger body shepherds the smaller body into a regular orbit. Alinda asteroids have a 3:1 orbital resonance with Jupiter and a 4:1 orbital resonance with the Earth.
4179 Toutatis/1989 AC is roughly peanut shaped, leading astronomers to suspect is a 'rubble-pile' type of object; two large rocks joined by smaller debris. It is classified as an S-type Asteroid, a stoney object made up mostly of magnesium-iron silicates. Such objects are typical of the inner Solar System, where icy or carbonaceous material is likely to have been evaporated away by the heat of the Sun.
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The orbit of 4179 Toutatis/1989 AC. Image created using the JPL Small-Body Database.
In addition to observations made from Earth, 4179 Toutatis/1989 AC will be the subject of a flyby by the China National Space Administration's Chang'e 2 probe, which has already taken a large number of high-resolution images of the Lunar surface.
See also The Geminid Meteors, The Leonid Meteors, Four more asteroids found to be co-orbitals of Neptune, Asteroid 2012 TC4 passes within 95 000 km of the Earth and Comet C/2012 S1 (ISON) to pass within 60 000 000 km of the Earth.
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