FOSSIL's preliminary thermal architecture
FOSSIL (FTS fOr CMB Spectral diStortIon expLoration) is a proposed ESA M8 mission tailored to measure the spectral distortions monopole of the Cosmic Microwave Background (CMB) with a sensitivity three orders of magnitude beyond the COBE/FIRAS legacy measurement. Achieving this sensitivity demands an extraordinarily challenging cryogenic architecture: the scientific instrument must be maintained at 4.5 K, while the detector focal plane assembly operates at 50 mK. This paper presents an overview of the preliminary thermal architecture of the FOSSIL payload, from the spacecraft service module at 293~K down to the sub-kelvin detector stage. The design draws on heritage from the Planck and ARIEL missions and relies on a staged passive cooling chain comprising a multi-layer insulation blanket, three V-groove radiators (operating at approximately 130 K, 90 K, and 50 K), and a 25 K actively cooled shield fed by an ESA-provided 4 K mechanical cryocooler. Sub-kelvin temperatures are achieved via a multi-stage adiabatic demagnetisation refrigerator (ADR) developed for NewAthena/X-IFU, providing continuous cooling at 1.8 K and 350 mK, and 50 mK with an 80% duty cycle. The Focal Plane Assembly (FPA), housing four Kinetic Inductance Detector (KID) arrays at 50 mK, is thermally isolated from the 4.5 K bench via a carbon-fibre reinforced polymer (CFRP) hexapod with staged heat interception. We outline the staged cooling concept and show that the architecture closes with positive thermal margins at every stage; the detailed steady-state thermal budget will be presented in a forthcoming dedicated paper.