  Upon discovery, asteroid (309239) 2007 RW10 was considered a Neptune Trojan
candidate. The object is currently listed by the Minor Planet Center as a
Centaur but it is classified as a Scattered Disk or Trans-Neptunian Object by
others. Now that its arc-length is 8,154 days and has been observed for more
than 20 years, a more robust classification should be possible. Here we explore
the orbital behaviour of this object in order to reveal its current dynamical
status; we perform N-body simulations in both directions of time to investigate
the evolution of its orbital elements. In particular, we study the librational
properties of the mean longitude that currently librates around the value of
the mean longitude of Neptune with an amplitude of nearly 50 degrees and a
period of about 7,500 years. Our calculations show that it has been in its
present dynamical state for about 12,500 years and it will stay there for
another 12,500 years. Therefore, its current state is relatively short-lived.
Due to its chaotic behaviour, the object may have remained in the 1:1 mean
motion resonance with Neptune for several 100,000 years at most, undergoing
transitions between the various resonant states. (309239) 2007 RW10 is
currently a quasi-satellite, the first object of this dynamical class to be
discovered around Neptune. With a diameter of about 250 km, it is the largest
known co-orbital in the Solar System. Its significant eccentricity (0.30) and
orbital inclination (36 degrees), strongly suggest that it did not form in situ
but was captured, likely from beyond Neptune. With an apparent magnitude of
21.1 at opposition (October), it is well suited for spectroscopic observations
that may provide information on its surface composition and hence eventually
its origin.
