FOSSIL's preliminary thermal architecture

FOSSIL (FTS fOr CMB Spectral diStortIon expLoration) is a proposed ESA M8 mission designed to measure the spectral distortion monopole of the Cosmic Microwave Background with sensitivity three orders of magnitude better than the COBE/FIRAS legacy measurement. This sensitivity places extreme demands on the cryogenic system: the scientific instrument must be held at 4.5 K while the detector focal plane assembly operates at 50 mK.

The preliminary thermal architecture, which draws on heritage from Planck and ARIEL, uses a staged passive cooling chain beginning at the 293 K service module: a multi-layer insulation blanket, three V-groove radiators 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. To get below 4 K, a multi-stage adiabatic demagnetisation refrigerator (ADR) developed for NewAthena/X-IFU provides continuous cooling at 1.8 K and 350 mK, and reaches 50 mK with an 80% duty cycle. The Focal Plane Assembly, which houses 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.

The paper outlines the staged cooling concept and reports that the architecture closes with positive thermal margins at every stage; the full steady-state thermal budget is planned for a forthcoming dedicated paper. This demonstrates the feasibility of the required cryogenic chain for the proposed mission.