STE-QUEST
The Space-Time Explorer and QUantum Equivalence Principle Space Test (STE-QUEST) was a candidate mission for the M3 launch opportunity in ESA's Cosmic Vision programme. The mission was designed to answer a range of questions in fundamental physics by performing precision measurements with high accuracy atomic sensors. In particular, STE-QUEST would test Einstein's Equivalence Principle, which underpins the theory of General Relativity; explore the boundaries with the quantum world; and search for new fundamental constituents and interactions in the Universe. In February 2014 the PLATO mission was selected for the M3 launch opportunity.