The Black Hole Explorer Cryocooling Instrument

The Black Hole Explorer (BHEX) is a space-based very-long baseline interferometry (VLBI) mission aimed at precision black hole measurements, detecting the photon ring around black holes, exploring spacetime, spin, and mass properties, and validating predictions of General Relativity. These objectives are achieved using cryogenic receivers with quantum-limited sensitivities across a broad frequency range. Dual-band receivers at 80-106 GHz and 240-320 GHz require operating temperatures of 20 K and 4.5 K, respectively. A cryocooling system with two cold stages will be employed: a 20 K stage handling a 125 mW heat load and a 4.5 K stage handling a 10 mW heat load. To design the cryocooling system, the mission leverages existing space industry technology at high Technology Readiness Levels (TRLs), informed by missions such as Planck, JEM/SMILES, Hitomi, and XRISM, and advancements from the ACTDP/JWST program. Integrating the cryocooler with the receivers and broader instrument involves careful consideration of thermal challenges, including linking the cold ends of each cooling stage to minimize heat losses and ensuring adequate passive cooling for the cryocooler warm end heat rejection. Key challenges and trade-offs include sizing the mass and reducing power consumption while meeting the receiver cold temperature requirements, which impact the scientific objectives. This paper addresses efforts to balance the scientific requirements with the limitations of technical cryocooling capabilities within the framework of a small-class (SMEX) space mission, presenting an overview of cooling needs, initial design considerations, a survey of 4 K spaceflight cryocooler developments, and trade-offs.