  THESEUS is a space mission concept, currently under Phase A study by ESA as
candidate M5 mission, aiming at exploiting Gamma-Ray Bursts for investigating
the early Universe and at providing a substantial advancement of
multi-messenger and time-domain astrophysics. In addition to fully exploiting
high-redshift GRBs for cosmology (pop-III stars, cosmic re-ionization, SFR and
metallicity evolution up to the "cosmic dawn"), THESEUS will allow the
identification and study of the electromagnetic counterparts to sources of
gravitational waves which will be routinely detected in the late '20s / early
'30s by next generation facilities like aLIGO/aVirgo, LISA, KAGRA, and Einstein
Telescope (ET), as well as of most classes of X/gamma-ray transient sources,
thus providing an ideal synergy with the large e.m. facilities of the near
future like, e.g., LSST, ELT, TMT, SKA, CTA, ATHENA. These breakthrough
scientific objectives will be achieved by an unprecedented combination of
X/gamma-ray monitors, providing the capabilities of detecting and accurately
localize and kind of GRBs and may classes of transient in an energy band as
large as 0.1 keV - 10 MeV, with an on-board NIR telescope providing detection,
localization (arcsec) and redshift measurement of the NIR counterpart. A Guest
Observer programme, further improving the scientific return and community
involvement is also envisaged. We summarize the main scientific requirements of
the mission and provide an overview of the updated concept, design (instruments
and spacecraft) and mission profile.
