Radiometer effect in space missions to test the equivalence principle

Experiments to test the equivalence principle in space by resting the universality of free fall in the gravitational field of the Earth have to take into account the radiometer effect, caused by temperature differences in the residual gas inside the spacecraft as it is exposed to the infrared radiation from Earth itself. We report the results of our evaluation of this effect for the three proposed experiments currently under investigation by space agencies: mu SCOPE, STEP, and GG. It is found that in mu SCOPE, which operates at room temperature, and even in STEP, where the effect is greatly reduced by means of Very low temperatures, the radiometer effect is a serious limitation to the achievable sensitivity. Instead, by axially spinning the whole spacecraft and with an appropriate choice of the sensitivity axes-as proposed in GG-the radiometer effect averages out and becomes unimportant even at room temperature.