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).
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]