The heating of a spinning artificial satellite by natural radiation
sources such as the Sun and the Earth results into temperature
gradients arising across the satellite's surface. The corresponding
anisotropic emission of thermal radiation leads to a recoil force,
commonly referred to as ``thermal force''. We have developed a
quantitative theory of this effect, based on more general assumptions
than used so far to model such radiation forces on LAGEOS--like
satellites. In particular, our theory holds for any ratio of the
three basic time scales of the problem: the rotation period of the
satellite, the orbital period around the Earth, and the relaxation
time for the thermal processes. Thus, we avoid the simplifying
assumption of a comparatively fast rotational motion, which will fail
for LAGEOS within the next decade, owing to magnetic dissipation
effects. A number of predictions about the future behaviour of
non--gravitational long--term orbital perturbations of LAGEOS become
possible with the new theory. Here we have studied in particular the
Yarkovsky--Schach thermal force effects arising as a consequence of
the solar radiation flux onto the satellite, periodically interrupted
by eclipses. Starting on about year 2005, the orbital perturbation
effects predicted by the new theory are substantially different from
those inferred in the fast--rotation case. This holds not only for
the long--term semimajor axis effects, but also for eccentricity and
inclination perturbations.
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