  The Yarkovsky (orbital drift) and YORP (spin state change) effects play
important roles in the dynamical and physical evolution of asteroids.
Thermophysical modelling of these observed effects, and of thermal-infrared
observations, allows a detailed physical characterisation of an individual
asteroid to be performed. We perform a detailed physical characterisation of
near-Earth asteroid (1620) Geographos, a potential meteor stream source and
former spacecraft target, using the same techniques as previously used in
Rozitis et al. (2013) for (1862) Apollo. We use the advanced thermophysical
model (ATPM) on published light-curve, radar, and thermal-infrared observations
to constrain the thermophysical properties of Geographos. The derived
properties are used to make detailed predictions of the Yarkovsky orbital drift
and YORP rotational acceleration, which are then compared against published
measurements to determine Geographos's bulk density. We find that Geographos
has a thermal inertia of 340 +140/-100 J m-2 K-1 s-1/2, a roughness fraction of
>50%, and a bulk density of 2100 +550/-450 kg m-3 when using the
light-curve-derived shape model with the radar-derived maximum equatorial
diameter of 5.04 +/- 0.07 km. It is also found that the radar observations had
overestimated the z-axis in Geographos's shape model because of their
near-equatorial view. This results in a poor fit to the thermal-infrared
observations if its effective diameter is kept fixed in the model fitting. The
thermal inertia derived for Geographos is slightly higher than the typical
values for a near-Earth asteroid of its size, and its derived bulk density
suggests a rubble-pile interior structure. Large uncertainties in shape model
z-axes are likely to explain why radar and thermal-infrared observations
sometimes give inconsistent diameter determinations for other asteroids.
