True. After sending out a photon of energy E, a spaceraft of mass m will have its (invariant!) mass reduced by a factor of sqrt(1 - 2E/mc²). In terms of energi

True. After sending out a photon of energy E, a spaceraft of mass m will have its (invariant!) mass reduced by a factor of sqrt(1 - 2E/mc²). In terms of energies, this is all rather trivial conceptionally: Rest energy (aka 'mass') gets converted into the kinetic energies of the spacecraft and the photon.