The European Space Agency (ESA) has selected ARRAKIHS as its second FAST-class (F2) scientific mission, with a view to launching it in 2030. The mission will observe faint stellar structures surrounding nearby galaxies, in order to better understand the formation and evolution of galaxies. The University of Liège is participating at both the technical and scientific levels, with the involvement of the Centre Spatial de Liège (CSL) and the Multi-wavelength, Extragalactic and Galactic Astrophysics (MEGA) laboratory.
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RRAKIHS (Analysis of Resolved Remnants of Accreted galaxies as a Key Instrument for Halo Surveys), selected by the ESA in 2022, aims to observe the diffuse and very faint stellar halos surrounding galaxies similar to our own. These structures bear the traces of successive mergers with small satellite galaxies. Under the influence of gravity, these dwarf galaxies break apart and form stellar streams, which have already been detected in the Milky Way by the Gaia mission, ESA’s astrometry satellite launched in 2013. By studying a sample of around a hundred nearby galaxies, up to a few tens of millions of light-years away, ARRAKIHS will enable us to test the standard model of galaxy formation, according to which large galaxies are built up through the gradual accretion of smaller galaxies.
ESA’s "Fast" class missions are characterised by rapid development - taking less than ten years from selection to launch - and by a lightweight satellite (weighing less than 1,000 kg) placed in orbit as a piggyback payload on a larger mission. ARRAKIHS is the second mission in this series, hence its designation ‘F2’, the first having been Comet Interceptor (launch in 2029).
The main challenge for ARRAKIHS lies in the fact that the structures to be observed, located on the outskirts of galaxies, are extremely faint, much fainter than the central disc where most of the stars are concentrated. To detect them, the instrument must not be dazzled by unwanted light from neighbouring sources such as the Sun, the Earth or the Moon. Reducing this straylight is such a critical requirement that it dictates the entire design of the instrument. This is precisely where the expertise of the University of Liège and the CSL comes into play.
The CSL and MEGA are involved
The Centre Spatial de Liège (CSL) is providing ULiège’s technical contribution to the mission. As a member of the instrument consortium, coordinated primarily by Spain and Switzerland, the CSL is responsible for assembling the Straylight Baffle, the instrument designed to reduce unwanted light from the Sun, the Earth and the Moon. This subsystem comprises two black-coated baffles and a protective cover, which is also useful for calibration. The CSL will develop the opening mechanism, which must operate between −120 °C and +70 °C and on which the smooth running of the mission directly depends, and will also carry out the analysis of stray light for the entire payload. This work draws on experience from previous missions, such as COROT and CHEOPS, and has led to the establishment of a dedicated measurement facility. Two prototypes have already been built and tested in a vacuum, with the opening of the hatches having been validated at extreme temperatures. "“Our responsibility covers both the rejection of stray light and the lid-opening mechanism, on which the success of the mission directly depends,” explains Etienne Lallemand, a cryogenics and mechanical engineer responsible for technical oversight at the CSL. “The challenge is to ensure its reliability over a very wide temperature range, drawing on the experience gained from previous missions.”
On the scientific front, the MEGA laboratory (STAR Research Institute / Faculty of Sciences) will analyse the mission’s data. “ARRAKIHS will enable us to observe, around a hundred nearby galaxies, the faint structures left behind by the merger of small galaxies,” says Michaël De Becker, an astrophysicist and the project’s scientific lead at ULiège. “This is a direct test of the galaxy formation model, complementing what we are studying with EUCLID, and forms part of our laboratory’s multi-wavelength approach.” The EUCLID mission observes distant galaxies, whilst ARRAKIHS focuses on nearby objects, paying particular attention to stray light. Its ability to image regions of weak emission in the visible and near-infrared spectrum is also of interest to other research being carried out within MEGA in the inner regions of our galaxy, complementing observations conducted in other parts of the spectrum, from radio to high energies.
Finally, the mission is of interest for training purposes, as students on the Master’s programme in Space Sciences at ULiège have already taken part, through work placements, in the CSL’s work on the optical and baffle prototypes, and others may contribute to future activities.
With its approval, ARRAKIHS moves from the study phase into the construction phase. For the University of Liège, this combines an instrumental contribution, led by the CSL, with scientific operations, carried out by the MEGA laboratory, illustrating its strong ties to space science and instrumentation.
The ARRAKIHS mission (ARRAKIHS Mission Consortium, AMC) is being developed by ESA, and the consortium is led by Prof. Rafael Guzmán (Instituto de Física de Cantabria – IFCA, CSIC–UC, Spain). Scientific coordination is provided by Rebekka Coles-Bieri (University of Zurich, UZH). The consortium brings together more than 250 scientists and engineers from seven ESA Member States, led by Spain: Austria, Belgium, Norway, Portugal, Sweden and Switzerland. Contributions are also made by institutions and companies from the United Kingdom, France, Denmark, the Netherlands, the United States, Taiwan and Thailand.
For Belgium, the University of Liège is involved via the Liège Space Centre (CSL) and the MEGA laboratory (STAR Research Institute), whilst the University of Ghent (UGent) has been invited to participate in the scientific aspect of the project. The Belgian contributions are supported by BELSPO and ESA’s PRODEX programme.
Five years on from the July 2021 floods, four studies carried out by engineers at the Hydraulics in Environmental and Civil Engineering (HECE) laboratory at the University of Liège shed new light on these extreme floods.
L'édition 2026 de la Nuit des Chercheur·es se tiendra le vendredi 25 septembre 2026. Impliquée dans la Nuit des Chercheur·es depuis sa création, l'ULiège vous invite à participer à cette manifestation de culture scientifique.
nGHOST2Market to track stray light from space telescopes and GrAVITe, an optical gyroscope to stabilise high-precision instruments.
