ESA's newly adopted F class mission, set for launch by 2030, is the first space telescope designed specifically to image the dim stellar streams that wrap around galaxies, offering a direct empirical check on prevailing models of how galaxies and the dark matter around them came…
For decades, galaxy formation has been told as a story written in two kinds of evidence. One is the deep, sharp imagery of galaxies themselves. The other is the cold arithmetic of dark matter in the cosmic web. What sits between them, the actual record of how galaxies grew, has been almost invisible from the ground. The European Space Agency's Arrakihs mission, adopted this week, is the first instrument designed specifically to make that record legible. Its job is to test, on a sample large enough to matter, whether the prevailing picture of how galaxies assembled through mergers actually holds, or whether the Milky Way turns out to be a special case that the model was quietly built around.
A galaxy's halo is the dim shell that surrounds the bright disk. Most of its mass is dark matter. A thin frosting of stars and hot gas is spread through it, often in long streams that are the leftover trails of smaller galaxies absorbed long ago. Reading those streams is, in principle, the cleanest way to reconstruct a galaxy's merger history. In practice, the stellar component is so faint, orders of magnitude dimmer than the night sky above even a good observatory, that ground-based surveys have only ever mapped the halo of one galaxy in detail: our own, mostly because we live inside it. The faint outskirts of other galaxies have, as the Arrakihs factsheet puts it, remained largely outside empirical reach. ESA's framing is that the mission is the first space mission specifically designed for low surface brightness astronomy.
That is what makes the adoption notable. The agency's Science Programme Committee, meeting in Tenerife on 10 to 11 June 2026, signed off on a mission to map the stellar haloes of at least 80 Milky-Way-mass galaxies, not to celebrate a result, but to commit to building the tool that might let researchers argue from real halo data instead of simulation forecasts. Arrakihs is the second F-class, or "fast," mission of ESA's Cosmic Vision programme, after Comet Interceptor. F-class missions must complete within a decade of selection; Arrakihs was selected in November 2022 and is planned for launch by the end of 2030. Adoption means the study phase is finished and ESA is now committed to implementation: spacecraft build, integration, and test.
The instrument is one binocular assembly of two pairs of small telescopes, four cameras in total, covering near-ultraviolet through visible light into the near-infrared. The factsheet puts the instrument mass at under 160 kilograms, and the near-infrared detector heritage comes from Webb and Euclid. The spacecraft bus is adapted from existing low-Earth-orbit platforms, with the pointing stability and thermal control tightened for astronomy. Nominal science life is three years, with an extension possible.
The lead institution is Spain, with core partners in Switzerland, Austria, Belgium, Norway, Portugal, and Sweden, and instrument work led by the Spanish firm Satlantis, with contributions channelled through ESA's Prodex programme. As Professor Carole Mundell, ESA's Director of Science, said in the adoption release, the mission is a "ground-breaking and unique galactic archaeology mission" that "showcases the flexibility and breadth of ESA's Science Programme."
The honest question the mission is built to address is whether standard galaxy-formation and dark-matter models survive contact with a representative sample of haloes. As Mundell noted in the same release, "we haven't been able to study enough of them to be sure that our models of galaxy formation, and by extension the role of dark matter, are correct." Arrakihs is not designed to prove anything. It is designed to convert a long-standing model assumption, that haloes everywhere look broadly like ours, into something that can be tested against real data, on a sample that includes our galaxy as one case among many rather than as the template.
That is also where the uncertainty lives. Halo light is faint by definition, and most of what Arrakihs will detect depends on the same kind of simulation forecasts that produced the predictions it is meant to test. The mission has not yet flown. A successful adoption is a commitment to build, integrate, and test a specific spacecraft, not a guarantee that the data will resolve the question. What adoption does guarantee, by the end of the 2020s, is a sample of halo maps at a sensitivity no previous instrument has aimed for. The argument over what they show will be the next decade's work.