Euclid is the second medium-class mission (M2) of ESA’s Cosmic Vision mandatory science programme, an astronomy and astrophysics mission selected by the agency’s Science Programme Committee (SPC) on 4 October 2011 and adopted on 20 June 2012. It is focused chiefly on cosmology, seeking to study the origin, nature, structure and evolution of the universe, and to advance our understanding of dark energy and dark matter, two components of the cosmos still shrouded in mystery.

The Euclid satellite, for which Thales Alenia Space Italy in Turin is the prime contractor, is scheduled to depart Earth in the second half of 2023. The mission is setting out to map vast sections of the cosmos in an attempt to learn more about the origin and evolution of our universe. The mission’s two main goals are to understand why the universe is expanding at an accelerating rate under the effect of this mysterious “dark” energy, and to map the equally mysterious “dark” matter, which while it remains invisible to the naked eye and to instruments contributes with visible matter—stars, nebula, etc.—to the gravitational effects that bind stars together into galaxies, and galaxies into clusters. By peering ever farther back in time, Euclid will be attempting to reconstruct how our universe has evolved over the last ten billion years under the opposing effects of dark matter and dark energy.

During its nominal six-year mission, Euclid plans to survey about a third of the sky, that is, a little under 15,000 square degrees, the remainder being hidden by the galactic plane—the disk in which stars in the Milky Way lie—and by the ecliptic plane—the disk in which the planets in our solar system lie. This survey will be complemented by observations that will go approximately ten times deeper pointed at three fields near the ecliptic poles, one to the north spanning 20 square degrees and two to the south each spanning 10 square degrees. These fields will be regularly visited throughout the mission and will serve to calibrate data and verify the stability of the telescope and instruments, as well as to acquire science data for observing galaxies and the most distant quasars in the universe.

Euclid will thus survey billions of galaxies and the evolution of large-scale cosmic structures as far back as ten billion years, in the visible and near-infrared (from 550 to 2,000 nm). To do this, it plans to determine the redshift (z) of observed sources using spectrometry and photometry instruments, complemented for photometric measurements by telescopes on Earth operating in the visible portion of the spectrum.

Covering such a vast area of sky and with its catalogues of billions of stars and galaxies, the mission will be acquiring science data of value well beyond the field of cosmology. This record of data will provide the global astronomy community with an abundance of sources for decades to come, constituting a repository of new astronomic objects for study by telescopes such as JWST, E-ELT, TMT, ALMA, SKA and the Vera C. Rubin Observatory.

To accomplish this gigantic mapping effort, Euclid will be carrying two instruments: the Near Infrared Spectro Photometer (NISP), operating in the visible spectrum, and the VISible Instrument (VIS), both developed by an international consortium led by France, through the IAP astrophysics institute in Paris (IAP/CNRS). The Euclid consortium comprises more than 2,200 people (425 in France) working at some 250 research laboratories (40 in France) across 16 countries.