Test of the equivalence principle in space

Summary Any violation of the equivalence principle (EP) between test masses in the near-Earth orbit is about 500 times bigger than on the ground, which makes the case for a space experiment very strong. Indeed, ESA and NASA (the European Space Agency and the American National Aeronautics and Space Administration) are currently studying at Phase A level the space mission STEP, whose main goal is to test the universality of free fall to 1 part in 10¹⁷ by means of a combination of very advanced technologies (drag free with proportional thrusters, superfluid-He temperature, SQUID sensors). We discuss the key features of STEP as well as some novel ideas about the possibility of testing the equivalence principle at room temperature in a non-drag-free satellite. Paper presented at the 6th Cosmic Physics National Conference, Palermo, 3–7 November 1992.