ESA adopts galactic archaeology mission Arrakihs

Being adopted means that the study phase is complete, the mission is shown to be feasible, and ESA commits to implementing it. In the upcoming development phase, the spacecraft and its scientific instrumentation will be built, integrated and extensively tested.  

Arrakihs is the second ‘fast’ or F-class mission of ESA’s Cosmic Vision programme, needing less than ten years from its selection in November 2022 until launch. The decision to adopt it was made at the Science Programme Committee meeting at the Instituto Astrofísico de Canarias, Tenerife (10–11 June 2026). 

“Arrakihs is a ground-breaking and unique galactic archaeology mission. By uncovering hard-to-see galaxy haloes, it will reveal new details of how galaxies form and whether the Milky Way galaxy is unique. Its rapid development showcases the flexibility and breadth of ESA’s Science Programme,” says Professor Carole Mundell, ESA’s Director of Science. 

The mission name stands for Analysis of Resolved Remnants of Accreted galaxies as a Key Instrument for Halo Surveys.

Uncovering galactic history 

Picture a galaxy. You might imagine a glowing, spiralling disc of stars, gas and dust. What you probably don’t picture is the much larger ball-shaped region surrounding this disc, filled with matter that is much harder to see. 

This region, called the galaxy halo, is mostly made up of invisible dark matter acting as the galaxy’s gravitational glue. The rest of the halo consists of normal matter, including stars and hot, charged gas. Arrakihs will observe diffuse stellar haloes and structures such as stellar streams – remnants of small galaxies that were torn apart by gravity. 

Importantly, a galaxy’s halo contains clear traces of how the galaxy formed and evolved over cosmic time. Scientists believe that galaxies grow over time by merging with others. Because galaxy haloes are so faint, 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.  

By mapping stellar streams, Arrakihs will allow us to piece together the history of past mergers and give an estimate of the number of ‘lonely’ stars that were ripped away from their galaxies during mergers. 

In total, Arrakihs plans to investigate at least 80 galaxies with a similar mass to the Milky Way galaxy. This is a large enough number to get statistics on how a ‘typical’ galaxy forms, allowing us to understand how unique our home galaxy is.

Two pairs of European eyes

The mission needs to detect extremely faint objects with a ‘low surface brightness’. To do this, Arrakihs will carry one scientific instrument consisting of two pairs of ‘binocular telescopes’, a total of four cameras. Each camera is sensitive to a different band of wavelengths, spanning from the near-ultraviolet, through the visible spectrum, into the near-infrared.  

The instrument is being designed and developed by a consortium of ESA Member States led by Spain. Other core consortium partners are Switzerland, Austria, Belgium, Norway, Portugal and Sweden. Many of the instrument contributions are supported through ESA’s Prodex programme. 

Arrakihs will join ESA’s fleet of Cosmic Observers. These missions primarily address two top-level science themes of ESA’s Cosmic Vision 2015–2025, namely: What are the fundamental physical laws of the Universe? and How did the Universe originate and what is it made of? 

For more information, please contact   
ESA Media Relations   
media@esa.int