The Giotto ion mass spectrometer

An ion mass spectrometer for the Giotto mission is proposed. A wide range of ion species and velocity distributions are expected to be found in the interaction between solar wind and cometary plasmas. The resultant demands on instrument dynamic range preclude use of a single sensor for measurements of ion composition. The Giotto ion mass spectrometer consists of two sensors: one optimized for the outer and the other for the inner coma, with each obtaining complementary information in the region for which it is less than optimal. Both sensors have mass resolution m/Delta m = 20 and use two-dimensional detector arrays to improve time resolution. Considerable onboard data compression is used to reduce the four-dimensional measurements (mass, energy, elevation and azimuth) to the available data rate.