Thermo-physical properties of 162173 (1999 JU3), a potential flyby and rendezvous target for interplanetary missions

Context. Near-Earth asteroid 162173 (1999 JU3) is a potential flyby and rendezvous target for interplanetary missions because of its easy-to-reach orbit. The physical and thermal properties of the asteroid are relevant for establishing the scientific mission goals and also important in the context of near-Earth object studies in general. Aims. Our goal was to derive key physical parameters such as shape, spin-vector, size, geometric albedo, and surface properties of 162173 (1999 JU3). Methods. With three sets of published thermal observations (ground-based N-band, Akari IRC, Spitzer IRS), we applied a thermophysical model to derive the radiometric properties of the asteroid. The calculations were performed for the full range of possible shape and spin-vector solutions derived from the available sample of visual lightcurve observations. Results. The near-Earth asteroid 162173 (1999 JU3) has an effective diameter of 0.87 +/- 0.03 km and a geometric albedo of 0.070 +/- 0.006. The chi(2)-test reveals a strong preference for a retrograde sense of rotation with a spin-axis orientation of lambda(ecl) = 73 degrees, beta(ecl) = -62 degrees and P-sid = 7.63 +/- 0.01 h. The most likely thermal inertia ranges between 200 and 600 Jm(-2) s(-0.5) K-1, about a factor of 2 lower than the value for 25143 Itokawa. This indicates that the surface lies somewhere between a thick-dust regolith and a rock/boulder/cm-sized, gravel-dominated surface like that of 25143 Itokawa. Our analysis represents the first time that shape and spin-vector information has been derived from a combined data set of visual lightcurves (reflected light) and mid-infrared photometry and spectroscopy (thermal emission).