  Thermal inertia determines the temperature distribution over the surface of
an asteroid and therefore governs the magnitude the Yarkovsky effect. The
latter causes gradual drifting of the orbits of km-sized asteroids and plays an
important role in the delivery of near-Earth asteroids (NEAs) from the main
belt and in the dynamical spreading of asteroid families. At present, very
little is known about the thermal inertia of asteroids in the km size range.
Here we show that the average thermal inertia of a sample of NEAs in the
km-size range is 200 $\pm$ 40 J m&#8722;2 s&#8722;0.5 K&#8722;1. Furthermore,
we identify a trend of increasing thermal inertia with decreasing asteroid
diameter, D. This indicates that the dependence of the drift rate of the
orbital semimajor axis on the size of asteroids due to the Yarkovsky effect is
a more complex function than the generally adopted D^(&#8722;1) dependence, and
that the size distribution of objects injected by Yarkovsky-driven orbital
mobility into the NEA source regions is less skewed to smaller sizes than
generally assumed. We discuss how this fact may help to explain the small
difference in the slope of the size distribution of km-sized NEAs and main-belt
asteroids.
