Euclid (space telescope)

Euclid is an ESA-led cosmology mission with contributions from NASA. Its central goal is to examine how the universe's expansion and the growth of cosmic structure have changed over time, helping constrain the properties of dark energy and dark matter. These are inferred through their gravitational and cosmological effects; the telescope does not directly photograph either substance.

The observatory surveys the sky in visible and near-infrared wavelengths. It combines sharp optical imaging, near-infrared imaging and photometry, and near-infrared spectroscopy. Galaxy shapes provide data for weak gravitational-lensing studies, in which the subtle distortion of distant galaxies' images traces the distribution of matter along the line of sight. Galaxy positions and redshift estimates help map how structure is distributed at different cosmic epochs. Together these methods support measurements of cosmic geometry and expansion history.

Euclid's planned wide survey covers about 14,000 square degrees—more than one-third of the extragalactic sky—and aims to observe billions of galaxies, reaching objects whose light has travelled for as long as about 10 billion years. A complementary deep survey repeatedly observes selected fields to detect fainter and more distant objects. The resulting catalogue and calibrated observations are also useful beyond the mission's principal dark-universe questions, including studies of galaxies, clusters and transient or moving objects.

Euclid launched from Cape Canaveral on 1 July 2023 aboard a SpaceX Falcon 9 and travels around the Sun–Earth L2 region, about 1.5 million kilometres from Earth. It began routine science observations in February 2024. In March 2025, ESA released an early set of observations from the mission's three deep fields: the one-week dataset included images containing 26 million galaxies, with the most distant reported at more than 10.5 billion light-years away. Such early releases demonstrate survey capabilities; they are not, by themselves, final measurements resolving the nature of dark energy or dark matter.