Optimized cooling system for balloon-borne astronomy: the FIREBall-2 solution

The Faint Intergalactic-medium Redshifted Emission Balloon (FIREBall-2) instrument is an ultraviolet multi-object slit spectrograph co-funded by the French Centre National d’Études Spatiales and the US National Aeronautics and Space Administration. Deployed via a stratospheric balloon, its goal is to observe the circumquasar and circumgalactic medium in emission. To detect this low surface brightness emission, the instrument is equipped with a photon-counting detector that requires low dark current via low-temperature operation, necessitating the overall instrument to be placed within a vacuum vessel. We detail the development, implementation, and performance of the specialized cooling system engineered to support FIREBall-2’s critical subsystems, including the detector controller, cryocooler, and guider camera, in the extreme conditions of stratospheric ballooning. The cooling system uses a closed-loop fluid circuit, combined with an ice fusion and water boiling mechanism to dissipate ∼35 MJ of heat across an environment temperature span from −80°C to +40°C and pressure variations from 1 atm to 3 hPa. By circulating coolant through a reservoir of 10 kg of crushed ice, the system can ensure temperature stability during the 24-h mission. Thermal and thermohydraulic analysis, coupled with ground-based testing, confirms the system’s effectiveness in meeting the cooling requirements. In-flight performance data further validate the system’s ability to maintain operational stability in suborbital conditions, providing an efficient solution for future balloon-borne instruments.