Quasi-satellite of Earth - Asteroid 2002 AA29

Discovery

2002 AA29 is a small near-Earth asteroid that was discovered on January 9, 2002 by the LINEAR (Lincoln Near Earth Asteroid Research) automatic sky survey.

2002 AA29's mean orbital period about the Sun is exactly one sidereal year. This means that it is locked into a relationship with the Earth, since such an orbit is only stable under particular conditions. 2002 AA29 follows a so-called horseshoe orbit along the path of the Earth.

Only 5 asteroids of this sort are known, locked into a 1:1 resonance with the Earth. The first was 3753 Cruithne, discovered in 1986.

Stats

Diameter: ~0.06 km

Semi-major axis: 0.993 AU (same as Earth)

Rotation: 33 minutes

Minor planet category: Aten asteroid

(data from JPL Small-Body Database)

Orbit

2002 AA29's orbit lies for the most part inside the Earth’s orbit. From orbital disturbances by the gas giant planets, mainly Jupiter, and the Yarkovsky effect (force due to asymmetrical absorption and emission of infra-red radiation), asteroids are diverted into the inner Solar System, where their orbits further influenced by close approaches with the inner planets.

2002 AA29 has probably been brought in the same way from the outer Solar System into the Earth’s influence. However, it is also suggested that the asteroid has always been on a near-Earth orbit and thus that it or a precursor body was formed near Earth’s orbit. In this case one possibility is that it could be a fragment from a collision of a middle-sized asteroid with the Earth or the Moon.

The orbit of the asteroid is almost circular, with an eccentricity of 0.012 which is even lower than that of the Earth at 0.0167. At the time of its discovery the orbit of 2002 AA29 was unique, because of which the asteroid is often called the first true co-orbital companion of the Earth, since the paths of previously discovered asteroids are not very similar to the Earth’s orbit.

The very low orbital eccentricity of 2002 AA29 is also an indication that it must always have been on a near-Earth orbit, or the Yarkovsky effect must have comparatively strongly caused it to spiral into the inner Solar System over billions of years, since as a rule asteroids which have been steered by planets have orbits with higher eccentricity.

Physical nature

Relatively little is known about 2002 AA29 itself. With a size of about 50 to 110 metres it is very small, on account of which it is seen from the Earth as a small point even with large telescopes, and can only be observed using highly sensitive CCD cameras. At the time of its closest approach on 8 January 2003 it had an apparent brightness in the visible region of magnitude 20.4.

So far nothing concrete is known about the composition of 2002 AA29. Because of its nearness to the Sun, it cannot however consist of volatile substances such as water ice, since these would evaporate or sublime.

Presumably it will have a dark, carbon-bearing or somewhat lighter silicate-rich surface; in the former case the albedo would be around 0.05, in the latter somewhat higher at 0.15 to 0.25. It is due to this uncertainty that the figures for its diameter cover such a wide range.