  Testing the effects predicted by the General Theory of Relativity, in its
linearized weak field and slow motion approximation, in the Solar System is
difficult because they are very small. Among them the post-Newtonian
gravitomagnetic Lense-Thirring effect, or dragging of the inertial frames, on
the orbital motion of a test particle is very interesting and, up to now, there
is not yet an undisputable experimental direct test of it. Here we illustrate
how it could be possible to measure it with an accuracy of the order of 1%,
together with other tests of Special Relativity and post-Newtonian gravity,
with a joint space based OPTIS/LARES mission in the gravitational field of
Earth. Up to now, the data analysis of the orbits of the existing geodetic
LAGEOS and LAGEOS II satellites has yielded a test of the Lense-Thirring effect
with a claimed accuracy of 20%-30%.
