  Between 2010 and 2017 we have collected new optical and radar observations of
the potentially hazardous asteroid (2102)~Tantalus from the ESO NTT and Danish
telescopes at the La Silla Observatory and from the Arecibo planetary radar.
The object appears to be nearly spherical, showing a low amplitude light-curve
variation and limited large-scale features in the radar images. The spin-state
is difficult to constrain with the available data; including a certain
light-curve subset significantly changes the spin-state estimates, and the
uncertainties on period determination are significant. Constraining any change
in rotation rate was not possible, despite decades of observations. The convex
lightcurve-inversion model, with rotational pole at
${\lambda}=210{\pm}41${\deg} and ${\beta}=-30{\pm}35${\deg}, is more flattened
than the two models reconstructed by including radar observations: with
prograde (${\lambda}=36{\pm}23${\deg}, ${\beta}=30{\pm}15${\deg}), and with
retrograde rotation mode (${\lambda}=180{\pm}24${\deg},
${\beta}=-30{\pm}16${\deg}). Using data from WISE we were able to determine
that the prograde model produces the best agreement in size determination
between radar and thermophysical modelling. Radar measurements indicate
possible variation in surface properties, suggesting one side might have lower
radar albedo and be rougher at centimetre-to-decimetre scale than the other.
However, further observations are needed to confirm this. Thermophysical
analysis indicates a surface covered in fine-grained regolith, consistent with
radar albedo and polarisation ratio measurements. Finally, geophysical
investigation of the spin-stability of Tantalus shows that it could be
exceeding its critical spin-rate via cohesive forces.
