Saturday, 26 October 2013

Asteroids in retrograde orbits.

Almost all bodies in the Solar System orbit in the same direction that the Sun itself rotates, and which is presumed to be the direction in which the primordial disk from which the Solar System evolved rotated. The small number of bodies known to have retrograde orbits (i.e. rotate in the opposite direction to everything else) are thought to have begun life as Oort Cloud objects, and to have entered the inner Solar System on more-or-less random trajectories as a result of close encounters with other bodies within the Oort Cloud. However studies of other stellar systems have revealed the presence of planets in retrograde orbits, suggesting that this simple model may not always apply. Some of these planets are also in orbital resonances with other bodies in non-retrograde orbits. The orbital resonance is caused by an exchange of orbital momentum between the planets as they pass each other; this also means the orbits are not quite constant. At any given time one of these planets will be running ahead of its average orbit and the other behind. Each time they pass they exchange energy, so that the one that was running slow speeds up, and the one that was running fast slows down.

In a paper published on the online arXiv database at Cornell University Library on 1 August 2013 and submitted for publication in the Monthly Notes of the Royal Astronomical Society, Helena Morais of the Department of Physics at the University of Aveiro and Fathi Namouni of the Observatoire de la Côte d’Azur at the Université de Nice describe six bodies in our own Solar System which are in both retrograde orbits and orbital resonances with planets.

343158 (2009 HC82) is an Apollo Group Object, that crosses the Earth's orbit, but always remains inside that of Jupiter. It is tilted at a high angle to the plane of the Solar System, in a retrograde orbit with a 3:1 resonance with Jupiter (i.e. it completes three orbits about the Sun for every one of Jupiter's). Despite fairly frequent close encounters with Venus, Earth and Mars, this orbit appears to be reasonably stable.

The orbit of 343158 (2009 HC82). JPL Small Body Database Browser.

2005 VD is a Centaur Family Object (body orbiting between Jupiter and Neptune, with an orbit that crosses that of at least one planet), with a retrograde trajectory that keeps it inside the orbit of Saturn and crosses that of Jupiter at its closest to the Sun. It's orbit is inclined to the plane of the Solar System, but not as strongly as that of 343158 (2009 HC82). Because lies close the the plane of the Solar System, 2005 VD repeatedly has close encounters with both Jupiter and Saturn, this is calculated to give it a somewhat chaotic, unstable orbit, which Morais & Namouni calculate moves from a 7:11 resonance with Jupiter (in which 2005 VD completes 7 orbits for every 11 of Jupiter), to a 9:13 resonance (9 orbits of 2005 VD for every 13 of Jupiter), back to a 7:11 resonance, then to a 2:3 resonance (two orbits of 2005 VD for every 3 of Jupiter) then back to a 7:11 resonance and repeating the cycle.

The orbit of 2005 VD. JPL Small Body Database Browser. 

1999 LE31 is another Centaur, this time with a more inclined retrograde orbit that crosses those of both Saturn and Jupiter. It is close to being in a 1:2 resonance with Jupiter (completing 1 orbits for every 2 of Jupiter) and a 7:9 resonance with Saturn (completing 7 orbits for every 9 of Saturn), though Jupiter and Saturn are not in resonant orbits, so it is not possible to be in a resonant orbit with both. Morais & Namouni suggest that 1999 LE31 may move between these resonances.

The orbit of 1999 LE31. JPL Small Body Database Browser.

330759 (2008 SO218) is also a Centaur, with an inclined retrograde orbit that crosses those of both Saturn and Jupiter. It appears to be in a 1:2 resonance with Jupiter, and is also close to being in a 5:4 resonance with Saturn.

The orbit of 330759 (2008 SO18). JPL Small Body Database Browser.

2006 BZ8 is another Centaur, this time in a steeply inclined retrograde orbit that crosses the orbits of both Saturn and Jupiter, though its angle of inclination means that it never comes near Saturn. It's orbital period, 29.79 years, is close to that of Saturn, 29.46 years, and it appears to be in a 2:5 resonance with Jupiter.

The orbit of 2006 BZ8. JPL Small Body Database Browser. 

Finally 2009 QY6 is a Centaur in a steeply inclined retrograde orbit that crosses the orbits of Jupiter, Saturn and Uranus. It appears to be in a 2:3 resonance with Saturn and also to be close to being in a 2:1 resonance with Uranus. It is thought that repeated close encounters with Uranus make this orbit slightly unstable.

The orbit of 2009 QY6. JPL Small Body Database Browser.

See also The nature and history of 'Quasi-Hilda Object' 2000 YN30The Eulalia and Polana asteroid familiesFour more asteroids found to be co-orbitals of NeptuneNeptune's Trailing Trojans and The stability of Neptune's Trojan Asteroids.

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