The orbit analysis of LAGEOS satellite has resulted into the
discovery and/or reassessment of several very small
sources of perturbation on satellite orbits. The analysis of
orbital arcs of duration ranging from one month to several years has revealed
that perturbative effects are present, having unpredicted long--term or
secular components down to the $10^{-12}$ m/s$^2$ acceleration level. It
was soon realized that those perturbations have a non--gravitational
origin.
In recent years, we have devoted some effort to the physical modelling of
radiative perturbations, caused by momentum exchanges with an
appropriate radiative field, and have considered their potential role in
the analysis of LAGEOS orbit residuals. These perturbations include:
(i) direct solar radiation pressure; (ii) radiation pressure from the Earth's
emitted/reflected/diffused radiation flux; (iii) the so--called {\it
thermal thrust} force.
The main results of this work are reviewed, discussing its relationships
with models developed by other research groups. In particular,
we present a list of the physical processes which still appear to
need more detailed and realistic modelling to reach a better understanding
of LAGEOS dynamics at the $10^{-12}$ m/s$^2$ level.
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