Deep Space 1 encounter with Comet 19P/Borrelly: Ion composition measurements by the PEPE mass spectrometer

Abstract

We report in situ ion mass spectrometer measurements from the coma of Comet 19P/Borrelly, obtained during the flyby of the Deep Space 1 spacecraft on 22 September 2001. Cometary ions were detected at distances from ∼5.5 × 10 5 km to 2200 km from the nucleus of the comet. The predominant heavy ions observed during the seven minute interval about closest approach to Borrelly included OH + (at 57% of the total water‐group ion density), H 2 O + (at 29%), O + (at 13%), CH 3 + (at 5%), and CH 2 + (at 4%). Of particular note is the small amount of H 3 O + (<9%), as this was the most abundant molecular ion observed at closest approach to Comet Halley. This difference is due to the difference in water production rates between the two comets.