2007 RW10 - A Large Quasi-Satellite of Neptune

Quasi-satellites have been found around Venus, Earth, Jupiter and Saturn. Basically, a quasi-satellite is an object that co-orbits the Sun together with a planet in what is known as a 1:1 orbital resonance. This means that the orbit of a quasi-satellite has the same orbital period as the planet, although the orbital eccentricity of a quasi-satellite is usually greater. From the perspective of the planet, a quasi-satellite will appear to revolve around the planet even though a quasi-satellite is technically in orbit around the Sun.

2007 RW10 is a quasi-satellite of the planet Neptune. It has been a quasi-satellite of Neptune for ~12,500 years and will continue to do so for another ~12,000 years. 2007 RW10 co-orbits the Sun together with Neptune in a 1:1 orbital resonance. Although it orbits the Sun and not Neptune, 2007 RW10 appears to go around Neptune every 164.8 years (i.e. the duration of one Neptunian year) from the perspective of Neptune. 2007 RW10 can come as close as 0.86 AU to Neptune. That is close to Neptune’s Hill radius of 0.775 AU. To become a satellite of a planet, an object must have an orbit that lies within the planet’s Hill radius. With an estimated diameter of roughly 250 km, 2007 RW10 is probably the largest known object in a 1:1 orbital resonance with a planet.

The relative motion of 2007 RW10 with respect to Neptune over the next 3,000 years appears to trace a kidney-shaped path when viewed from Neptune. C. de la Fuente Marcos & R. de la Fuente Marcos (2012).

Reference:
C. de la Fuente Marcos & R. de la Fuente Marcos (2012), “(309239) 2007 RW10: a large temporary quasi-satellite of Neptune”, arXiv:1209.1577 [astro-ph.EP]