Euclid: investigating the secrets of dark matter and dark energy

One of the greatest mysteries in today’s cosmology is dark matter and dark energy. We know from observations that galaxies behave in a way that the gravitational interactions of stars alone cannot explain. As such, something else must be present. Scientists call these materials “dark matter” and “dark energy”, but this is where our knowledge stops. ESA’s Euclid mission aims to help clear up some of these mysteries.

Scientific objectives

Dark matter, while possessing mass, is invisible. Thus, like all objects with mass, dark matter bends the path of light coming next to it. As such, measuring this effect is the only way to observe dark matter. Dark matter is invoked to explain the rotational speed of stars around the centers of galaxies and makes up 95% of our universe. Dark energy, on the other hand, is used to explain why galaxies are getting apart at an accelerating rate, instead of slowing down due to gravity.

One of JWST's fist images, showing galaxies distorted by gravitational lensing. Image credit: NASA — *One of JWST’s first images, showing galaxies distorted by gravitational lensing. Credits: NASA*

Euclid will map the galaxies and their evolution over the past 10 billion years. By seeing how galaxies evolve over time, scientists hope to gain valuable insight into how dark matter and dark energy act, and ultimately their very nature.

The light emitted by an object moving away is shifted toward the red color due to the Doppler effect. The expansion of the universe also increases redshift. Since we know the universe is expanding at an accelerating rate, we can use the speed of a galaxy to determine its distance too, with faster galaxies being more distant.

The spacecraft

Euclid is approximately 4.7 m tall and 3.7 m in diameter and has a launch mass of 2 tons. At the base, there is an octagonal service module, containing propulsion, communication, and attitude control equipment. On top of that sits a telescope with a primary mirror diameter of 1.2 m. The telescope will use two scientific instruments: VIS and NISP. One side of the spacecraft is covered by a solar panel, which also acts as a Sun shield.

ESA's Euclid in its final assembly stages at the Thales Alenia Space plant in Turin. The solar array/sunshield is on the right, while the white cylinder is the telescope. Image credit: ESA — *ESA’s Euclid in its final assembly stages at the Thales Alenia Space plant in Turin. The solar array/sunshield is on the right, while the white cylinder is the telescope. Credits: ESA*

VIS (Visual Imaging Channel) is a visible spectrum camera that will image cosmic structures to measure their deformation due to dark matter. The detector contains around 600 megapixels. NISP (Near-Infrared Spectrometer and Photometer) is a sensor that will detect light in the near-infrared spectrum. NISP will measure the redshift and allow the mapping of galactical evolution. The spectrometer will also provide information on their chemical composition and allow scientists to measure the redshift more accurately.

Both instruments will operate at -180 °C to maximize accuracy. The payload module on which they are mounted is made of silicon carbide, whose use was pioneered by Herschel and Gaia missions. Unlike metal, silicon carbide experiences minimal dilation or contraction when the temperature changes. This allows the manufacturing of extremely precise optics. However, working with this material is very challenging, since it is extremely brittle. Special care was employed to ensure it isn’t damaged during the launch.

Euclid's NISP instrument, designed to measure the redshift of distant galaxies. Image credit: ESA — *Euclid’s NISP instrument, designed to measure the redshift of distant galaxies. Credit: ESA*

The mission

The spacecraft will be injected on a transfer trajectory toward the Sun-Earth L2 Lagrange point by a Falcon 9. Euclid will spend at least six years orbiting there, together with other spacecraft like JWST and GAIA. The mission lifetime is limited by the reserves of cold gas fuel needed to maintain the orbit. This is because orbits around this Lagrange point are naturally unstable and require constant course corrections. If enough propellant is left, the mission might be extended. The launch is scheduled to take place in September 2023.

History

The Euclid mission was conceived in 2007 as part of ESA’s Cosmic Vision program. It emerged from a combination of two proposals: Dark UNiverse Explorer (DUNE) and SPectroscopic All sky Cosmic Explorer (SPACE). It was selected as a medium-class mission in 2011. The scientific instruments were supplied by the Euclid Consortium, with members in 13 European countries, while the spacecraft was built by Thales Alenia Space. Originally, the launch vehicle was supposed to be a Russian-made Soyuz ST-B, but the war in Ukraine made that impossible. As such, the choice was made to use Falcon 9 instead.

Cosmic Vision is the third ESA space science campaign. It follows Horizon 2000 and Horizon 2000 Plus. Currently, the CHEOPS and Solar Orbiter missions are already underway, studying exoplanets and the Sun respectively. In the future, ESA will also launch the JUICE Jupiter orbiter, the PLATO and ARIEL exoplanet telescopes, the Athena X-ray telescope, and the LISA gravitational waves detector. But of course, the first mission coming up is Euclid.