Pathfinder: accuracy improvement of comet Halley trajectory for Giotto navigation

Abstract

The ‘Pathfinder’ project represents a cooperative effort involving the space agencies Intercosmos (Soviet Union), the National Aeronautics and Space Administration (NASA; United States) and the European Space Agency (ESA; Europe) for the benefit of the Giotto mission, the context of which was the stringent requirement for Giotto to fly past comet Halley at a distance of ∼500 km on the sunward side. The uncertainties^1–5 in the comet's trajectory were such that this requirement could not be fulfilled with the necessary accuracy using only ground-based astronomical observations; early studies^6–8,20 revealed that a considerable improvement could be attained by incorporating space-borne observations. As the two Vega spacecraft were to reach the comet a few days before Giotto, optical observations performed by the Vega cameras could be used for this purpose. These, combined with a very accurate determination of the Vega trajectories using VLBI (very-long-baseline interferometry)^9,10, permitted a final fly-by distance of 600km to be achieved with an uncertainty of only ±40 km (1σ probability level).