  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 in
2018 where it will perform a survey, land and obtain surface material, then
depart in Dec 2019 and return to Earth in Dec 2020. We observed Ryugu with the
Herschel Space Observatory in Apr 2012 at far-IR thermal wavelengths, supported
by several ground-based observations to obtain optical lightcurves. We
reanalysed previously published Subaru-COMICS and AKARI-IRC observations and
merged them with a Spitzer-IRS data set. In addition, we used a large set of
Spitzer-IRAC observations obtained in the period Jan to May, 2013. The data set
includes two complete rotational lightcurves and a series of ten
"point-and-shoot" observations. The almost spherical shape of the target
together with the insufficient lightcurve quality forced us to combine
radiometric and lightcurve inversion techniques in different ways to find the
object's key physical and thermal parameters. We find that the solution which
best matches our data sets leads to this C class asteroid having a retrograde
rotation with a spin-axis orientation of (lambda = 310-340 deg; beta = -40+/-15
deg) 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 in
the range 150 to 300 Jm-2s-0.5K-1. Based on estimated thermal conductivities of
the top-layer surface in the range 0.1 to 0.6 WK-1m-1, we calculated that the
grain sizes are approximately equal to between 1 and 10 mm. The finely
constrained values for this asteroid serve as a `design reference model', which
is currently used for various planning, operational and modelling purposes by
the Hayabusa2 team.
