STEP payload development

The Satellite Test of the Equivalence Principle, STEP, will test what is arguably the most fundamental principle of physics to unprecedented accuracy. The experiment will enhance our knowledge of the identity of mass and inertia by five or more orders of magnitude, to one part in 10 18, by comparing the rate of fall of test masses in earth orbit. The equivalence principle is the cornerstone of General Relativity, and detection of any violation would be an unambiguous sign of new physics. This paper describes progress toward the payload design for STEP.