Abstract :
STEP (Satellite Test of the Equivalence Principle) was studied by ESA at Assessment and Phase A level as a candidate for the second medium-size project (M2) and, after its non-selection, again at Assessment and Phase A level for M3. During the M2 cycle, STEP was studied as an ESA/NASA collaborative project, during the M3 cycle as a European-only project. During both cycles, the payload consisted not only of several differential accelerometers to test the Equivalence Principle, but also of accelerometers to measure the Constant of Gravity, to test the inverse square law of gravity, to search for a hypothetical force between spin-polarised and ordinary matter, and to perform a high-precision geodesy experiment. In the end, STEP was not selected as M3 either. The studies carried out by ESA for M3 complemented similar studies performed at about the same time by NASA and by CNES. Over the last couple of years a variety of mission options was studied by the three space agencies with the result that a low-cost, cryogenic option that exclusively focuses on the Equivalence Principle test is the best way forward. This mission (MiniSTEP) is now under study as a NASA/ESA collaborative project.
