The ion mass spectrometer on Giotto

Abstract

As expected, a wide range of ion species and velocity distributions were found as the Giotto spacecraft traversed the coma of Comet Halley. The outer coma was characterised by the interaction between solar wind and cometary plasmas; the inner coma by the outflow of cometary neutrals and their ionisation products. The resultant demands on instrument dynamic range had precluded the use of a single sensor for measurements of ion composition. The Giotto ion mass spectrometer (IMS) therefore consisted of two sensors: one optimised for the outer coma and the other for the inner coma, with each obtaining complementary information in the region for which it was not optimised. Both sensors featured mass-imaging characteristics, thereby permitting simultaneous measurements of several ion species by means of multi-detector arrays. Resultant mass per charge resolution was >or=20. In addition to mass per charge, the energy per charge and the elevation and azimuth of the incident ions were measured. In-flight data show that the IMS met its scientific goals for the Halley encounter.