STEP - A FUNDAMENTAL-PHYSICS LABORATORY IN-SPACE

Experiments in fundamental physics have been performed on the ground f or many centuries. Only recently, with ESA's Call for Mission Proposal s for the second medium-size project (M2) in 1989, have the possibilit ies of space flight been offered to the scientific community of fundam ental physicists in Europe. This was done realising that many experime nts on the ground are limited by environmental disturbances (e.g. micr oseismicity) and that space offers the possibility of improving these experiments by typically several orders of magnitude. STEP - Satellite Test of the Equivalence Principle - is a 'fundamental-physics laborat ory in space', combining four different experiments in one payload. Th e latter consists of eight differential accelerometers accommodated in a quartz block and cooled to 2 K in order to: test the Equivalence Pr inciple, search for a new interaction between quantum-mechanical spin and ordinary matter, determine the Constant of Gravity G, and test the validity of the inverse square law of gravity. This payload will be f lown aboard a drag-free spacecraft to be launched in 2004 into a Sun-s ynchronous, circular orbit at 350 - 400 km altitude. STEP also offers the possibility of a unique geodesy mission, as the orbit is drag-free and known with centimetre-precision and the STEP accelerometers can b e combined to form a highly sensitive gradiometer.