Radio occultation experiments with Comet Halley

Previous radio occultation investigations on cometary comae and tails have included refraction measurements and intensity scintillations of natural radio sources used to derive the density and structure of the cometary plasma. Significant improvements in the coverage and sensitivity of these measurements will be achieved during the present apparition of Comet Halley. The comet missions GIOTTO and VEGA will also feature passive radio science expeirments designed to measure comet-induced Doppler shifts of the dual-frequency spacecraft signals during Halley flyby. A brief survey of these radio occultation measurement techniques and their application in the specific case of Comet Halley are presented.