Star tracker design considerations for the Europa Orbiter mission

Star trackers are an accurate device used by most spacecraft to autonomously determine their orientation. The future Europa Orbiter mission poses significant environmental challenges not typically encountered in other space missions. The Jovian system in which the star tracker must operate is characterized by extremely intense ionizing radiation, which is primarily due to high-energy electrons and protons trapped by Jupiter´s magnetic field. The design of a star tracker that will operate in this environment must overcome many challenging problems. Ionizing radiation will cause ordinary optical glass to darken. Silicon based detectors typically experience threshold shifts, increased dark current, reduced quantum efficiency and reduced charge transfer efficiency (in the case of CCDs). An even larger challenge is posed by the charged particle flux impinging on the detector. Charged particles incident on any glass in the optical path will generate luminescence and Cerenkov radiation, which degrades the signal to noise ratio. This paper discusses issues related to the design of a star tracker that can operate in the Europa radiation environment