As project scientist for the unmanned Voyager spacecraft missions to the outer Solar System since 1972, and a major spokesman for the Voyager science team, he became especially well-known to the public in the 1980s. He has since been principal investigator on nine NASA spacecraft missions and coinvestigator on five more.

He was the P.I. for the Cosmic Ray System on Voyager 1 and 2 spacecraft, an experiment to measure cosmic rays.