January 30, 2019



Science Objectives

Euclid is an ESA mission to map the geometry of the dark Universe. The mission will investigate the distance-redshift relationship and the evolution of cosmic structures by measuring shapes and redshifts of galaxies and clusters of galaxies out to redshifts ~2, or equivalently to a look-back time of 10 billion years. It will therefore cover the entire period over which dark energy played a significant role in accelerating expansion.Proportions of Ordinary matter, Dark matter, Dark energy

Until about 30 years ago, astronomers thought the Universe was composed almost entirely of ordinary matter: protons, neutrons, electrons and atoms. In the intervening years, the emerging picture has changed dramatically. It is now assumed that ordinary matter makes up only about 5% of the Universe, and that its mass-energy budget is actually dominated by two mysterious components: dark energy and dark matter.

Dark energy, which accounts for the vast majority (69%) of the energy density of the Universe, is causing the expansion of the Universe to accelerate. The existence and energy scale of dark energy cannot be explained with our current knowledge of fundamental physics.

The remaining 26% of the energy density is in the form of dark matter which, like ordinary matter, exerts a gravitational attraction, but unlike normal matter does not emit light. The nature of dark matter is unknown, although several candidates are predicted by supersymmetric extensions of the standard model of particle physics. Plausible candidates for cold dark matter are the axion and the lightest supersymmetric particle; massive neutrinos can account for the hot dark matter. One possibility to explain one or both of these puzzling components is that Einstein's theory of general relativity, and thus our understanding of gravity, needs to be revised on cosmological scales. Together, dark energy and dark matter pose some of the most important questions in fundamental physics today.

Euclid is optimized for two primary cosmological probes:

  1. Weak gravitational Lensing (WL): Weak lensing is a method of mapping dark matter and gauging dark energy by measuring the distortions of galaxy images by mass inhomogeneities along the line of sight.
  2. Baryonic Acoustic Oscillations (BAO): BAOs are wiggle patterns imprinted in the structure of the Universe that provide a standard ruler to measure dark energy and the expansion of the Universe.

Weak gravitational lensing requires extremely high image quality because possible image distortions due to the optical system must be suppressed or calibrated out to be able to measure the true distortions by gravity.

Illustrations of the effect of a lensing mass on a circularly symmetric image and in galaxy cluster Abell 1689
Illustrations of the effect of a lensing mass on a circularly symmetric image.
In galaxy cluster Abell 1689, strongly lensed arcs can be seen around the cluster.
Every background galaxy is weakly lensed. Credit for Abell 1669:
NASA, ESA, and Johan Richard (Caltech, USA)

The Euclid baryonic acoustic oscillations experiment intends to determine the redshifts of galaxies to better than 0.1%, which can only be accomplished through spectroscopy.

 NASA, ESA, and R. Massey.
Illustration of weak gravitational lensing : the distribution of dark matter measured from the HST COSMOS survey using weak gravitational lensing. This survey covers an area similar to 8 times the size of the Moon. Euclid will produce higher resolution 3-D dark matter maps over the entire sky. Credit: NASA, ESA, and R. Massey.
 Galaxy clustering as a probe of the geometry of the universe. The same acoustic features (BAO) seen in the CMB can be observed in the distribution of galaxies, providing a standard cosmological ruler.
Illustration of the baryonic acoustic oscillations: Galaxy clustering as means of probing the geometry of the Universe. The same acoustic features (BAOs) seen in the cosmic microwave background (CMB) can be observed in the distribution of galaxies, providing a standard cosmological ruler.

Surveyed in the same cosmic volume, these techniques not only provide systematic cross-checks but also a measurement of large-scale structure via different physical fields (potential, density and velocity), which are required for testing dark energy and gravity on cosmological scales.

With its wide-field capability and high-precision design, Euclid will:

  1. Investigate the properties of dark energy by accurately measuring both the acceleration and the variation of acceleration at different ages of the Universe
  2. Test the validity of general relativity on cosmic scales
  3. Investigate the nature and properties of dark matter by mapping the three-dimensional dark matter distribution in the Universe
  4. Refine the initial conditions at the beginning of our Universe, which seeded the formation of the cosmic structures we see today.

Euclid is therefore poised to uncover new physics by challenging all sectors of the cosmological model. The Euclid survey can thus be thought of as the low-redshift, three-dimensional analogue and complement to the map of the high-redshift Universe provided by ESA's Planck mission.

Additional science with Euclid

Euclid will produce a massive legacy of deep images and spectra over at least half of the entire sky. This will be a unique resource for the astronomical community and will impact upon all areas of astronomy. Euclid's spatial resolution of 0.2 arcseconds is only achievable from space, and is comparable to the Hubble Space Telescope. With Euclid, the majority of the new sources identified by future imaging observatories, from radio to X-rays, will be readily associated to a known redshift, out to a redshift z~2. This adds enormously to the science return of these other projects, as it eliminates the time-consuming phase of redshift follow-up. Euclid will be a discovery machine on an unprecedented scale, and may well feed more detailed studies both with ground-based facilities and future satellites.


Euclid will be injected into a direct transfer orbit by a Soyuz ST 2-1b launch vehicle departing from Europe's spaceport in Kourou, French Guiana. The transfer phase to the target orbit around the second Sun-Earth Lagrange point, L2, will last approximately 30 days. A correction manoeuvre will be performed about two days into the flight, once sufficient tracking data have been acquired to evaluate the radial velocity error, which arises due to uncertainties in the conditions during launch. Commissioning of the spacecraft and instruments will start during the transfer phase. No insertion manoeuvre is needed to reach the chosen orbit.

A large-amplitude (~1x106 km) halo orbit around L2 has been selected because it offers optimum operating conditions for Euclid: a benign radiation environment, which is necessary for the sensitive detectors, and very stable observing conditions, sufficiently far away from the disturbing Earth-Moon system. In addition, the amount of propellant necessary is very favourable compared to alternative orbits.

To cope with the unprecedented science data volume generated by Euclid at L2, K-band (25.5-27 GHz) communications will be used, which offer a transfer rate of 55 Mbitps. As a consequence of the large variations in the Sun-spacecraft-Earth angle, a two-degrees-of-freedom mechanism for the antenna is needed to maintain the science telemetry link to Earth.

The selected orbit is eclipse-free. Trajectory-correction manoeuvres will be performed every 30 days.

Euclid sky-scanning strategy

Euclid sky-scanning strategyEuclid will survey the sky in 'step-and-stare' mode: the telescope will realize imaging and spectroscopic measurements by successive adjacent pointing of ~0.5 deg².

The sky coverage strategy is driven by the wide-survey requirement to cover 20,000 deg² of extragalactic sky during the mission lifetime of 5 years. The main considerations driving the survey strategy are:

  • The L2 orbit and spacecraft viewing constraints; the Sun-spacecraft line will turn 1 degree per day in the plane of the ecliptic.
  • Maintaining the thermal stability of the spacecraft constrains the pointing direction to be as perpendicular as possible to the Sun-spacecraft axis. This means that on a given day in the mission the viewing area is mostly confined to a great circle perpendicular to the Sun-spacecraft axis.
  • The fundamental exposure times of the instruments and the size of a field, which is 0.5x1.0 deg².

For the imaging channels, dithering is required to over-sample the PSF (point-spread-function) to fill the gaps between the detectors, and to ensure that the field is completely covered.

On a daily basis, Euclid will observe strips of adjacent sky fields along a great circle of (roughly) constant ecliptic longitude. A strip of  the order of 15-20 degrees long can be covered, depending on the geometry of the instruments' field of view and the integration time per field. During the assessment study, a field of 0.5 degree and an exposure time per field of 2,400 s has been assumed, in which case the daily coverage would be a strip of about 18 degrees.

On approximately a 3-4 week basis a patch will be observed: this is a square area of about 400 deg². The geometry of a patch is roughly 20x20 deg², but significant deviations are expected depending on the precise ecliptic of galactic latitude of a patch. After 6 months, the spacecraft pointing direction will be flipped (along a great ecliptic circle) to observe patches in the opposite hemisphere.

The extragalactic sky is presently defined by the regions covering latitudes over 30 degrees. The alignment of the galactic plane with respect to the ecliptic plane causes the extragalactic sky to be poorly accessible during the equinoxes (i.e. around 21 March and 21 September).

The deep survey will cover ~40 deg², and consists of patches of at least 10 deg² which are about two magnitudes deeper than the wide survey. The deep survey is obtained by regular visits to the same areas on the sky at regular intervals during the mission. The same observing mode as for the wide survey is used to monitor the temporal stability of the system.