The Haloes That Remember

The Milky Way has been eating smaller galaxies for billions of years, and the leftovers still hang in the sky. They are not dramatic, glowing arcs. They are extraordinarily faint ribbons of stars, gravitationally stretched out of dwarf galaxies that the Milky Way consumed long ago. Astronomers call them stellar streams, and they are the closest thing the galaxy has to a written history. ESA's newly adopted Arrakihs mission will spend years mapping that history around roughly 80 other large galaxies, building the first statistical record of how spirals like the Milky Way grow by absorbing their neighbors.

Arrakihs is a small, fast-built mission with an unusually specific job. The ESA Science Programme Committee formally adopted it at a meeting on 10–11 June 2026 in Tenerife, Spain, moving it from a study phase into implementation. Adoption means the agency has judged the mission feasible, has signed off on its design, and has committed to flying it. The target launch window is the end of 2030.

The name is an acronym for Analysis of Resolved Remnants of Accreted galaxies as a Key Instrument for Halo Surveys, which is closer to a thesis statement than a name. The idea is simple in principle and hard in practice. Around almost every large galaxy there is a roughly spherical halo of stars, mostly stripped from satellites that fell in and were torn apart. The visible spiral disc is only the bright inner part of a much larger structure. The halo is enormous, mostly empty, and extraordinarily dim relative to the night sky and the host galaxy. To see it at all requires a telescope designed for the faintest possible light, staring for long periods at the diffuse outskirts of other galaxies.

That is what Arrakihs is. According to ESA's release on the adoption, the payload is a single scientific instrument built from two binocular telescopes, four cameras in total, each sensitive to a different band spanning near-ultraviolet, visible, and near-infrared light. The mission is the second F-class, or "fast," selection of ESA's Cosmic Vision programme, chosen in November 2022 and intended to reach launch in under a decade from selection. The instrument consortium is led by Spain, with core partners in Switzerland, Austria, Belgium, Norway, Portugal, and Sweden, and additional contributions supported through ESA's Prodex programme.

The scientific question Arrakihs is built to answer is not new, but the data it will return is. Astronomers already know, from work in and around the Milky Way and Andromeda, that stellar haloes preserve a record of past mergers. What they do not yet know is whether the Milky Way's particular history is typical of large spirals, or whether it is the product of an unusually quiet or unusually violent past. A single galaxy, or even a handful, cannot answer that. Halo photometry is technically demanding, and current samples are small. Arrakihs is designed to push the sample size to roughly 80 Milky Way–mass galaxies, with deep, wide-field surface photometry tuned to the lowest surface brightnesses. That is the kind of dataset that turns a handful of illustrative case studies into a population, and it is the central promise the Universe Today writeup of the mission highlights.

Director of Science Carole Mundell called Arrakihs a unique galactic-archaeology mission in ESA's adoption statement, and pointed to the rapid development timeline as a showcase of the Science Programme's flexibility. The next phase is the unglamorous one: build, integrate, and extensively test the spacecraft and instruments before they are cleared for flight. F-class missions are designed to move fast, but "by the end of 2030" is a plan, not a manifested launch slot, and the development phase is where schedule risk lives.

What to watch is straightforward. If Arrakihs flies on schedule and its cameras deliver the surface-brightness sensitivity the science case depends on, the field will get its first large, uniform sample of halo structures around Milky Way–type galaxies. That sample will let researchers ask, with statistical confidence, whether the architecture of the Milky Way is the rule or the exception among large spirals. The Milky Way has been digesting its neighbors for most of the age of the universe. Arrakihs is being built to read the receipts.