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The JAXA Hayabusa-2 mission was approved in 2010 and launched on December 3,2014. The spacecraft will arrive at the near-Earth asteroid 162173 Ryugu in2018 where it will perform a survey, land and obtain surface material, thendepart in Dec 2019 and return to Earth in Dec 2020. We observed Ryugu with theHerschel Space Observatory in Apr 2012 at far-IR thermal wavelengths, supportedby several ground-based observations to obtain optical lightcurves. Wereanalysed previously published Subaru-COMICS and AKARI-IRC observations andmerged them with a Spitzer-IRS data set. In addition, we used a large set ofSpitzer-IRAC observations obtained in the period Jan to May, 2013. The data setincludes two complete rotational lightcurves and a series of ten"point-and-shoot" observations. The almost spherical shape of the targettogether with the insufficient lightcurve quality forced us to combineradiometric and lightcurve inversion techniques in different ways to find theobject's key physical and thermal parameters. We find that the solution whichbest matches our data sets leads to this C class asteroid having a retrograderotation with a spin-axis orientation of (lambda = 310-340 deg; beta = -40+/-15deg) in ecliptic coordinates, an effective diameter (of an equal-volume sphere)of 850 to 880 m, a geometric albedo of 0.044 to 0.050 and a thermal inertia inthe range 150 to 300 Jm-2s-0.5K-1. Based on estimated thermal conductivities ofthe top-layer surface in the range 0.1 to 0.6 WK-1m-1, we calculated that thegrain sizes are approximately equal to between 1 and 10 mm. The finelyconstrained values for this asteroid serve as a `design reference model', whichis currently used for various planning, operational and modelling purposes bythe Hayabusa2 team.

Hayabusa-2 Mission Target Asteroid 162173 Ryugu (1999 JU3): Searching for the Object's Spin-Axis Orientation