Having already conducted measuresment of Comets Encke and Giacobini-Zinner, the still-functioning Pioneer Venus Orbiter, launched in 1978, becomes the centerpiece of the American effort to gather close-up data on Halley’s Comet. Imaging the comet in ultraviolet light, Pioneer Venus captures Halley from a unique vantage point: both the comet and the planet being orbited by Pioneer are on the opposite side of the sun from Earth, when Halley is at its closest approach to the sun – an event that no other spacecraft is in a position to witness. The Pioneer Venus data will become the most critical data gathered on Halley by an American spacecraft after a planned March 1986 Space Shuttle mission to image the comet from Earth orbit is scrapped in the wake of the Challenger disaster in January.
