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Space-Time Explorer and QUantum Equivalence Principle Space Test (STE-QUEST) is a M-class mission recently proposed within the Cosmic Vision programme of the European Space Agency and selected as one of the candidate missions for a flight opportunity in 2022-2024. STE-QUEST is a fundamental physics mission designed to test the Einstein Equivalence Principle in space: An atomic clock of high stability and accuracy will measure the gravitational time dilation effect while an atom interferometer will compare the free fall of two rubidium isotopes. The STE-QUEST clock operates with cold Rb atoms. Its design and technology is based on a modified version of the PHARAO clock, presently being developed in the frame of the ACES mission. The on-board clock signal is specified to a fractional frequency instability of 3E-14/Sqrt(tau), where tau is the integration time, and inaccuracy of 1E-16. High performance links, both in the microwave and optical domain, will compare the space clock with clocks on the ground. STE-QUEST will take advantage of a highly elliptic orbit around the Earth to measure the clock red-shift effect to an inaccuracy of 2E-7 (4E-8 as a goal). Common-view comparisons of terrestrial clocks located in different continents will also measure the periodic effect of the gravitational frequency shift induced by the Sun to 2E-6 (6E-7 as a goal). Concurrently to the clock measurements, the two-isotope interferometer will monitor the differential acceleration between two samples of ultra-cold 85Rb and 87Rb atoms. The atom interferometry measurements will be particularly important around perigee, where the signal-to-noise ratio of a possible violation of the Weak Equivalence Principle can be maximized. The instrument sensitivity is specified to test the universality of the free propagation of matter waves to an uncertainty in the Eötvös parameter better than 1E-15. STE-QUEST is currently in the assessment phase, which will define the design of the mission, the spacecraft, its instruments and subsystems. This paper will discuss the STE-QUEST scientific objectives and present the first results of the study activities.

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