The ion mass spectrometer aboard the Giotto spacecraft measured the composition and velocity distributions of cometary ions at distances of∼7.5×l0⁶ to∼1,300 km from the nucleus of comet Halley. Well outside the bow shock, pick-up cometary H⁺ ions were found in a diffuse shell-like distribution. Heavier ions (C⁺, H₂O⁺-group, CO⁺ and S⁺) with similar distributions have been identified at ≤3×l0⁵ km. Solar-wind He²⁺ was found throughout the coma to as close as∼5,000 km; He⁺ produced by charge exchange was seen inside∼2×10⁵ km. Deeper within the coma the main cometary hot-ion species identified were H⁺, H₂⁺, C⁺, O⁺, OH⁺, H₂O⁺, H₃O⁺, CO⁺ and S⁺. A pile-up of heavy cometary ions was found at∼10⁴ km from the nucleus. Giotto crossed the contact surface at∼4,600 km, based on changes in ion flow velocity and temperature. Inside, ion temperatures as low as∼340 K and outflow velocities of∼1 km s⁻¹ were found. Outside the contact surface ion densities vary asr⁻², with a transition to anr^−l dependence approximately at the contact surface. A large C⁺ abundance throughout the coma indicates an unexpected direct source of atomic carbon. The nitrogen abundance, on the other hand, is relatively low.

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