| Mission Type: | Space observatory |
| Operator: | European Space Agency |
| Power: | 115 W |
| Launch Date: | 2037 (proposed)[1] |
| Launch Rocket: | Vega-E (baseline) |
| Orbit Regime: | Low Earth orbit |
| Orbit Altitude: | 600 km |
| Orbit Inclination: | 5° (equatorial) |
| Programme: | Cosmic Vision M-class |
| Previous Mission: | ARIEL |
Transient High-Energy Sky and Early Universe Surveyor (THESEUS) is a space telescope mission proposal by the European Space Agency that would study gamma-ray bursts and X-rays for investigating the early universe.[2] [3] If developed, the mission would investigate star formation rates and metallicity evolution, as well as studying the sources and physics of reionization.
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 other finalist was EnVision, a Venus orbiter. The winner, EnVision, was selected in June 2021 for launch in 2031.[5]
In November 2023, following a new selection process (2022) and a Phase-0 study (2023), THESEUS was selected by ESA for a new 2.5 year Phase-A study as one of the three candidates M7 missions (together with M-Matisse and Plasma Observatory).
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. This can provide fundamental information on the cosmic star formation rate, the number density and properties of low-mass galaxies, the neutral hydrogen fraction, and the escape fraction of ultraviolet photons from galaxies.[2]
The conceptual payload of THESEUS includes:[2]