The THESEUS mission

THESEUS is a mission concept under study by the European Space Agency (ESA) since 2018 as candidate for a medium-size mission within the Cosmic Vision ProgrammeTHESEUS was selected by ESA in 2023 to enter a new assessment phase study within the M7 opportunity (part of banner image credits: Loeb 2006 & ESO).


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An artist impression of the THESEUS spacecraft.


The mission is designed to vastly increase the discovery space of the high energy transient phenomena over the entirety of cosmic history. Its primary scientific goals will address the Early Universe ESA Cosmic Vision themes “How did the Universe originate and what is made of?” (4.1, 4.2 and 4.3) and will also impact on “The gravitational wave Universe” (3.2) and “The hot and energetic Universe” themes. This is achieved via a unique payload providing an unprecedented combination of: 1) wide and deep sky monitoring in a broad energy band (0.3 keV - 20 MeV); 2) focusing capabilities in the soft X-ray band providing large grasp and high angular resolution; and 3) on board near-IR capabilities for immediate transient identification and redshift determination.

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Left: long gamma-ray bursts in the cosmological context (adapted from the NASA/WMAP team). Right: short gamma-ray bursts as electromagnetic counter-parts of the gravitational wave signal emitted by merging neutron stars (credits: LIGO/VIRGO collaboration).


The foreseen payload of THESEUS includes the following instrumentation:

  • Soft X-ray Imager (SXI, 0.3 – 5 keV): a set of 2 lobster-eye telescopes units, covering a total field of view (FOV) of ~0.5sr with source location accuracy < 1-2’;
  • InfraRed Telescope (IRT, 0.7 – 1.8 μm): a 0.7m class IR telescope with 15’x15’ FOV, for fast response, with both imaging and spectroscopy capabilities;
  • X-Gamma rays Imaging Spectrometer (XGIS, 2 keV – 20 MeV): a set of 2 coded-mask cameras using monolithic X-gamma rays detectors based on bars of Silicon diodes coupled with CsI crystal scintillator, granting a ~2sr FOV and a source location accuracy of ~10 arcmin in the 2-150 keV, as well as a >4sr FoV at energies >150 keV.

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Sketches of the THESEUS satellite as provided by Airbus (left) and THALES (right) during the 2018-2021 study phase (credits: ESA).


The mission profile includes: an onboard data handling units (DHUs) system capable of detecting, identifying and localizing likely transients in the SXI and XGIS FOV; the capability of promptly (within a few tens of seconds at most) transmitting to ground the trigger time and position of GRBs (and other transients of interest); and a spacecraft slewing capability of at least 6°/minute. The baseline launcher / orbit configuration is a launch with Vega-C to a low inclination low Earth orbit (LEO, ~600 km, <5°), which has the unique advantages of granting a low and stable background level in the high-energy instruments, allowing the exploitation of the Earth’s magnetic field for spacecraft fast slewing and facilitating the prompt transmission of transient triggers and positions to the ground.


The main scientific goals of the proposed mission are to:

  1. Explore the Early Universe (cosmic dawn and reionization era) by unveiling a complete census of the Gamma-Ray Burst (GRB) population in the first billion years. Specifically to:

  • Perform unprecedented studies of the global star formation history of the Universe up to z ~ 10 and possibly beyond;

  • Detect and study the primordial (pop III) star population: when did the first stars form and how did the earliest pop III and pop II stars influence their environments?

  • Investigate the re-ionization epoch, the interstellar medium (ISM) and the intergalactic medium (IGM) up to z ~ 8 - 10: how did re-ionization proceed as a function of environment, and was radiation from massive stars its primary driver? How did cosmic chemical evolution proceed as a function of time and environment?

  • Investigate the properties of the early galaxies and determine their star formation properties in the re-ionization era.


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THESEUS capabilities for the investigation of the Early Universe in terms of capability of detecting and autonomously identifying high-redshift GRBs in 3.45 years of net scientific observations (red dots) compared to what has been achieved in the last 15 years.


  1. Perform an unprecedented deep monitoring of the X-ray transient Universe in order to:

  • Locate and identify the electromagnetic counterparts to sources of gravitational radiation and neutrinos, which may be routinely detected in the late ‘20s / early ‘30s by next generation facilities like aLIGO/aVirgo, eLISA, ET, or Km3NET;

  • Provide real-time triggers and accurate (~1 arcmin within a few seconds; ~1’’ within a few minutes) locations of (long/short) GRBs and high-energy transients for follow-up with next-generation optical-NIR (E-ELT, JWST if still operating), radio (SKA), X-rays (ATHENA), TeV (CTA) telescopes;

  • Provide a fundamental step forward in the comprehension of the physics of various classes of Galactic and extra-Galactic transients, e.g.: tidal disruption events (TDE), magnetars /SGRs, SN shock break-outs, Soft X-ray Transients SFXTS, thermonuclear bursts from accreting neutron stars, Novae, dwarf novae, stellar flares, AGNs and Blazars;

  • Provide unprecedented insights into the physics and progenitors of GRBs and their connection with peculiar core-collapse SNe and substantially increase the detection rate and characterization of sub-energetic GRBs and X-Ray Flashes;

  • Fill the present gap in the discovery space of new classes of high-energy transient events, thus providing unexpected phenomena and discoveries.


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THESEUS capabilities in the field of time-domain and multi-messenger Astrophysics.


By satisfying the requirements coming from the above main science drivers, the THESEUS payload will also automatically enable excellent observatory science opportunities, including, e.g., performing IR observatory science, especially providing capability for response to external triggers, thus allowing strong community involvement. We remark that THESEUS has survey capabilities for high-energy transient phenomena complementary to the Large Synoptic Survey Telescope (LSST) in the optical. Their joint availability in the next decade would enable a remarkable scientific synergy between them.


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THESEUS expected synergies in the 2030's scenario.