THESEUS is a mission concept that would monitor transient events in the high-energy Universe across the whole sky and over the entirety of cosmic history.
In particular, it expects to make a complete census of gamma-ray bursts (GRBs) from the Universe's first billion years, to help understand the life cycle of the first stars.
[4] THESEUS would provide real-time triggers and accurate locations of the sources, which could also be followed up by other space- or ground-based telescopes operating at complementary wavelengths.
The concept was selected in May 2018 as a finalist to become the fifth Medium-class mission (M5) of the Cosmic Vision programme by the European Space Agency (ESA).
The space observatory would study GRBs and X-rays and their association with the explosive death of massive stars, supernova shock break-outs, black hole tidal disruption events, and magnetar flares.