page_title: 2002 AA29
section_title: Orbital data
hierarchical_section_title: 2002 AA29 / Orbit / Orbital data
caption_attribution_description: Orbits of 2002 AA29 and Earth around the Sun viewed vertical to the ecliptic. The graphic shows the relative positions of both bodies to each other in year 2001.
context_page_description: 2002 AA₂₉ is a small near-Earth asteroid that was discovered on January 9, 2002 by the LINEAR automatic sky survey. The diameter of the asteroid is only about 20–100 metres. It revolves about the Sun on an almost circular orbit very similar to that of the Earth. This lies for the most part inside the Earth's orbit, which it crosses near the asteroid's furthest point from the Sun, the aphelion. Because of this orbit, the asteroid is classified as Aten type, named after the asteroid 2062 Aten.
  A further characteristic is that its 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. As yet only a few asteroids of this sort are known, locked into a 1:1 resonance with the Earth. The first was 3753 Cruithne, discovered in 1986.
  Asteroids that have a 1:1 orbital resonance with a planet are also called co-orbital objects, because they follow the orbit of the planet. The most numerous known co-orbital asteroids are the so-called trojans, which occupy the L4 and L5 Lagrangian points of the relevant planet. However, 2002 AA₂₉ does not belong to these.
context_section_description: Shortly after the discovery by LINEAR, Scientists at the Jet Propulsion Laboratory (JPL), the Athabasca University (Canada), the Queen's University in Kingston (Ontario, Canada), the York University in Toronto and the Tuorla Observatory of the University of Turku in Finland determined the unusual orbit of 2002 AA₂₉, and through further observations at the Canada–France–Hawaii Telescope in Hawaii it was confirmed that:
  Its orbit lies for the most part inside Earth's orbit. The orbits of most asteroids lie in the asteroid belt between Mars and Jupiter. Through orbital disturbances by the gas giant planets, mainly Jupiter and the Kirkwood gaps, and through 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 are 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 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 Earth or the Moon.
  Its mean orbital period is one sidereal year. After it was diverted into the inner Solar System – or formed on a path near Earth's orbit – the asteroid must have been moved into an orbit corresponding with Earth. In this orbit it was repeatedly pulled by Earth in such a way that its own orbital period became the same as that of Earth. In the current orbit, Earth thus holds the asteroid in synchronicity with its own orbit.
  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. The other near-Earth asteroids have on average a significantly higher eccentricity of 0.29. Also, all other asteroids in 1:1 resonance with Earth known before 2002 have very strongly elliptical orbits – e.g. the eccentricity of (3753) Cruithne is 0.515. 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 Earth, since the paths of previously discovered asteroids are not very similar to 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.
  The orbital inclination with respect to the ecliptic (orbital plane of Earth) of 2002 AA₂₉ is a moderate 10.739°. Hence its orbit is slightly tilted compared with that of Earth.
caption: Orbits of 2002 AA29 and Earth about the Sun, looking down on the ecliptic plane from above the Sun's North Pole; Image: JPL
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