Illucidating the dark: the Euclid space mission

The fact that the universe is not just expanding, but that this expansion is accelerating, implies that the energy density of the universe is dominated by two main components, both of largely unknown nature.¹ 76% of the energy density is in the form of dark energy, which causes the expansion of the universe to accelerate. Another 20% is in the form of dark matter, which, just like normal matter, causes a gravitational acceleration, but, mysteriously, does not emit light. Obtaining a fuller understanding of dark energy and dark matter would have profound implications for the fundamentals of physics.¹ The quest for a deeper understanding of these components of the universe is a primary challenge to cosmologists.

Euclid is a spaceborne survey mission dedicated to investigating dark energy, dark matter, and gravity,^{2, 3} and is planned for launch in 2020. It is the only mission dedicated to looking at the dark universe. The mission will characterise the signatures of dark energy on the 3D distribution of cosmic structures, and will provide measurements with a precision that could not be achieved from the ground. Euclid will also have the ability to survey the sky at both visible and near-infrared (NIR) wavelengths, making it a wide-field visible/NIR space mission.

The mission will take six years to complete a wide survey, covering 1/3 of the sky, and a deep survey, covering only 1/1000 of the sky.⁴ Euclid will measure the shapes of 2 billion galaxies with a visual imager (VIS) filter. The apparent shapes of galaxies are distorted by gravitational deflection of light due to dark matter concentrations. Euclid will also measure galaxy clustering, which is the non-random distribution of galaxies in the universe that results from the force of gravity. Galaxy clustering is measured from the 3D position of galaxies using a spectroscopic survey of 50 million galaxies. The distances of galaxies will be derived by using flux ratios of each galaxy in the three Euclid NIR bands, and will be complemented by ground-based visible photometry.

Euclid will use a 1.2m Korsch telescope (see Figure 1), which is designed to provide a large field of view, and will meet the demanding image quality requirements. A dichroic beam splitter located at the telescope's exit pupil directs the light to a VIS and a near infrared spectrometer-photometer (NISP). Both instruments cover a common field of view of 0.54deg.

Figure 1. The mechanical architecture of the Euclid telescope. Six struts connect the secondary mirror (M2) mounted on a frame through spiders to the primary mirror (M1) optical bench. The upper part of the optical bench supports the M1 and the M2 structure. The lower part supports the other telescope optics and both the visual imager and the near infrared spectrometer-photometer instruments. The baseline 1.2m three mirror Korsch configuration provides sufficient degrees of freedom to achieve the required image scale and low distortion. Illumination by the sun is blocked by telescope baffles and a sunshield. (Reprinted with permission.³)

The VIS instrument⁵ consists of a CCD focal plane array (FPA), with one wide visible band spanning a range of 550–900nm, a shutter mechanism, and a calibration unit. The FPA supports 6×6 CCDs (4,000×4,000 pixels each) with 0.101arcsec pixel plate scale. An optical fine guidance sensor mounted near the VIS instrument is used to achieve a relative pointing stability to a fraction of a VIS pixel.

The NISP employs 4×4, 2k×2k pixels H2RG detectors with 0.3arcsec per pixel. It can be operated in either photometer or slitless spectrometer mode.⁶ In photometer mode, the NISP images the sky in three filterbands (Y, J, and H), covering the wavelength range 0.92–2.0μm. In the slitless spectrometer mode, the light is dispersed using grating prisms in the range 1.1–2.0μm at a spectral resolution λ/Δλ∼ 250.

Two concepts for the spacecraft have been designed by two independent industrial contractors (see Figure 2). The nominal downlink data transfer will be performed at a maximum data rate of 850Gbit/day.

Figure 2. Phase A studies of the Euclid spacecraft. (Left) Design led by Astrium GmbH (Germany). (Right) Design led by Thales Alenia Space Italy (Turin). The key features that they share are (1) the sunshield with solar arrays pointing towards the sun and a steerable high gain K band antenna mounted at the bottom; (2) the service module at the bottom (architecture derived from the European Space Agency's Herschel satellite), and (3) the payload module with baffled telescope truss and thermal radiators.³

The processing of the raw telemetry data into results will be carried out in the Euclid science ground segment (SGS), which consists of the science data centres (SDCs) and the operations centre in Madrid. The SDCs are in charge of developing and running the Euclid pipeline.⁷ The Euclid Consortium (EC) develops the scientific processing algorithms and is responsible for the delivery of the scientific data products. All SGS operations revolve around the public Euclid Mission Archive, a central storage of the data.

For the space segment, ESA will provide the spacecraft, the payload module, with the telescope, and the CCDs and NIR detectors. The EC is funded by the national funding agencies. It provides the VIS and NISP instruments, and key elements of the SGS. The Consortium is also involved in the assessment of the scientific requirements and performances and will lead the scientific exploitation of the mission. The EC is composed of about 1000 scientists residing in more than 100 laboratories from 13 European countries.

Euclid is the only endorsed space mission dedicated to observe the dark universe. Its outstanding performances will provide by 2025 the high precision data needed to pin down the nature of dark energy and dark matter.