  The thermal inertia of an asteroid is an indicator of the thermophysical
properties of the regolith and is determined by the size of grains on the
surface. Previous thermophysical modeling studies of asteroids have identified
or suggested that object size, rotation period, and heliocentric distance (a
proxy for temperature) as important factors that separately influence thermal
inertia. In this work we present new thermal inertias for 239 asteroids and
model all three factors in a multi-variate model of thermal inertia. Using
multi-epoch infrared data of a large (239) set of objects observed by WISE, we
derive the size, albedo, thermal inertia, surface roughness, and sense of spin
using a thermophysical modelling approach that doesn't require a priori
knowledge of an object's shape or spin axis direction. Our thermal inertia
results are consistent with previous values from the literature for similarly
sized asteroids, and we identify an excess of retrograde rotators among
main-belt asteroids < 8 km. We then combine our results with thermal inertias
from the literature to construct a multi-variate model and quantify the
dependency on asteroid diameter, rotation period, and surface temperature. This
multi-variate model, which accounts for co-dependencies between the three
independent variables, identified asteroid diameter and surface temperature as
strong controls on thermal inertia.
