  The mean force exerted upon a perfect mirror moving in vacuum in a two
dimensional spacetime has the same expression as the radiation reaction force
computed in classical electron theory. It follows that unacceptable runaway
solutions are predicted. We show that this instability problem does not appear
when partially transmitting mirrors are studied. The mechanical impedance
describing the mirror coupled to vacuum radiation pressure is computed
explicitly; recoil is neglected. It is found to be a passive function, so that
stability is ensured. This is connected to the fact that no energy can be
extracted from the vacuum state.
