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We have performed detailed thermophysical and dynamical modelling of the Jovian Trojan (1173) Anchises. Our results show that this is the most unusual object. By examining observational data of Anchises taken by  IRAS ,  Akari  and  WISE  at wavelengths between 11.5 and 60 μm, together with the variations in its optical light curve, we find that Anchises is most likely an elongated body, with an axis ratio,  a / b , of around 1.4. This results in calculated best‐fitting dimensions for Anchises of 170 × 121 × 121 km (or an equivalent diameter of 136 +18/−11 km). We find that the observations of Anchises are best fitted by the object having a retrograde sense of rotation, and an unusually high thermal inertia in the range 25–100 J m −2  s −0.5  K −1  (3σ confidence level). The geometric albedo of Anchises is found to be 0.027 (+0.006/−0.007). Anchises therefore has one of the highest published thermal inertias of any object larger than 100 km in diameter, at such large heliocentric distances, as well as being one of the lowest albedo objects ever observed. More observations (visual and thermal) are needed to see whether there is a link between the very shallow phase curve, with almost no opposition effect, and the derived thermal properties for this large Trojan asteroid. Our dynamical investigation of Anchises’ orbit has revealed it to be dynamically unstable on time‐scales of hundreds of millions of years, similar to the unstable Neptunian Trojans 2001 QR 322  and 2008 LC 18 . Unlike those objects, however, we find that the dynamical stability of Anchises is not a function of its initial orbital elements, the result of the exceptional precision with which its orbit is known. Our results are the first to show that a Jovian Trojan is dynamically unstable, and add further weight to the idea that the planetary Trojans likely represent a significant ongoing contribution to the dynamically unstable Centaur population, the parents of the short‐period comets. The observed instability (fully half of all clones of Anchises escape the Solar system within 350 Myr) does not rule out a primordial origin for Anchises, but, when taken in concert with the result of our thermophysical analysis, suggest that it would be a fascinating target for a future study.

(1173) Anchises – thermophysical and dynamical studies of a dynamically unstable Jovian Trojan