Evaluation of In-Orbit Thermal Performance of X-Ray Astronomy Satellite “Hitomi”

This study presents the evaluation results of the thermal performance for Japan Aerospace Exploration Agency’s X-ray astronomy satellite (Hitomi) ASTRO-H mission. Hitomi was successfully launched on 17 February 2016. The thermal design of the spacecraft and scientific instruments was challenging because the high-energy resolution and the sensitivity needed to achieve the desired scientific objectives required small thermal distortion and a strict temperature range, in spite of large heat dissipation. Hitomi housed four telescopes and six detectors. Each detector had its own radiator and heat pipes for heat dissipation. The most essential mission instrument, the soft X-ray spectrometer, had four loop heat pipes to transfer heat from two sets of compressors and the cold heads of two-stage Stirling coolers. Six fans were mounted close to six cryocoolers for ground cooling in the fairing until just before launch. The heat pipes were operated properly in orbit. Two loop heat pipes for the cryocooler compressors continued operating from the time of launch, thereby maintaining cryocooler temperatures within the allowable temperature range. Comparing the thermal analysis results and flight data, the difference between the predicted and measured temperatures was less than 4°C for more than 95% of all measured temperatures. The thermal control system functioned properly in both launch operation and in orbit, and the thermal mathematical model simulated the actual thermal design of Hitomi.