Arrakihs factsheet

Name: Analysis of Resolved Remnants of Accreted galaxies as a Key Instrument for Halo Surveys (Arrakihs)   

Planned launch: Late 2030  

Mission: Galactic archaeology, exploring faint light from nearby galaxy haloes to learn about how galaxies like our own form  

Mission type: Arrakihs is a ‘fast’ or F-class mission in ESA's Science Programme 

Status: In development. ESA adopted the mission on 10 June 2026.  

Science objectives:
Galaxies like the Milky Way are more than a dense disc of stars and gas. They are embedded in a much larger spherical halo, filled with matter that is much harder to see. Haloes are made up of invisible dark matter acting as the galaxy’s gravitational glue, hot gas, and a diffuse population of stars. 

Arrakihs is setting out to image the faint starlight coming from haloes around galaxies that are similar to the Milky Way. It is the first space mission specifically designed to target ‘low surface brightness’ (LSB) structures. LSB objects give out far less light per unit area than the background night sky.  

Importantly, the structure of a galaxy’s halo can reveal much about how the galaxy formed. For example, stellar streams are the remnants of smaller galaxies or star clusters that were torn apart while merging with the host galaxy.   

Arrakihs will dig up cosmic history and reveal how Milky-Way-like galaxies form. This addresses two core themes of ESA’s Cosmic Vision: What are the fundamental physical laws of the Universe? and How did the Universe originate and what is it made of? 

Key questions: 

1. How do Milky-Way-mass galaxies and their haloes assemble? 
2. How does the Milky Way galaxy compare to other galaxies? 
3. Which aspects of galaxy formation models need to be updated to match observations? 
4. What can we infer about dark matter from stellar streams?  

Spacecraft and instruments: 
The spacecraft will carry a single instrument consisting of two binocular telescopes. Each of the four cameras is sensitive to a different band of wavelengths, spanning from the near-ultraviolet, through the visible spectrum into the near-infrared. The instrument will weigh less than 160 kg. Arrakihs uses similar near-infrared light detectors as those used in the NASA/ESA/CSA Webb and ESA Euclid missions.  

The spacecraft design builds on existing low-Earth-orbit satellites, adapted to meet the stringent pointing stability and temperature requirements of astronomy observations.  

Lifetime: Three years of science operations, with possible extension 

Partnership: Arrakihs is an ESA mission with Member State contributions led by Spain. Other core consortium partners are Switzerland, Austria, Belgium, Norway, Portugal and Sweden. The Spanish company Satlantis is leading the development of the scientific instrument. Many of the instrument contributions are supported through ESA's Prodex programme. 

History:  ESA launched a call for a ‘fast’ or F-class mission in December 2021. F-class missions need no more than ten years of development. These missions ensure the flexibility of ESA’s Science Programme. Arrakihs was selected among many proposals during the Science Programme Committee (SPC) meeting in November 2022, was adopted at the SPC meeting in June 2026, and is planned to launch by the end of 2030.  

Links to other missions:  
Arrakihs will complement results from ESA’s Euclid mission, launched in 2023. Euclid is mapping the Universe to uncover the role of gravity and the nature of dark matter and dark energy. Arrakihs will observe the faint haloes around nearby galaxies, which could indirectly teach us about dark matter. 

ESA’s Gaia mission has revealed unprecedented details of the Milky Way’s structure and history. Arrakihs is setting out to study a statistically relevant group of nearby Milky-Way-like galaxies to uncover whether similar galaxies all formed the same way. 

Arrakihs mission facts

• All satellite galaxies observed by Arrakihs belong to the Local Group, the cluster of galaxies we belong to  
• Arrakihs can observe astronomical objects more than a 1000 times dimmer than our eyes can see on a moonless night from a dark part of Earth  
• Arrakihs will observe the Universe in near-ultraviolet, visible and near-infrared light