The Space-Time Explorer and QUantum Equivalence Principle Space Test (STE-QUEST) was proposed as an M-class mission in response to the 2010 Call for Medium-size Missions for ESA's Cosmic Vision plan.
The mission concept arose from a consultation process with the scientific community that was carried out in 2009 by the ESA-appointed "Fundamental Physics Roadmap Advisory Team" (FPR-AT).
In February 2011, STE-QUEST was one of four missions (the other missions being EChO, LOFT, and MarcoPolo-R) recommended by the ESA advisory structure and selected for an assessment study; these missions were later joined by PLATO. The assessment phase for STE-QUEST started in April 2011 and concluded in mid-2013.
In the meantime, STE-QUEST was studied at the ESA Concurrent Design Facility (CDF) at the European Space Research and Technology Centre (ESTEC) in May 2011. The study concluded with the identification of a preliminary design of the mission and its payload.
The results of the CDF study have been used to prepare an open Invitation to Tender (ITT) to Industry. The ITT process closed in late 2011; the two parallel contracts arising from the ITT covered a 1-year assessment study of the STE-QUEST mission and were completed in 2012. In parallel, instrument studies made progress in terms of design, interfaces and resources consolidation, and performance budget analysis. To date, computational tools are available to verify traceability and compliance of the science requirements to mission objectives. Industry and Instrument Consortia activities concluded in the summer of 2013 with the delivery of the documentation due for the Preliminary Requirements Review.
In February 2014, PLATO was selected as M3, the third medium-class mission in ESA's Cosmic Vision Programme.