With a special interest in isotopic abundances, he pursued and promoted a wide range of observational and theoretical investigations toward understanding the origins of all the planets and small bodies of the Solar System. As one of the world's leading planetary scientists, he was an active participant in a great number of missions, including Apollo 15 and 16, Viking, Voyager, Galileo, Rosetta, Juno, and Cassini-Huygens. He played a leading role in the development of the Cassini-Huygens mission as a joint project of NASA and ESA, and was called upon frequently to promote this and other missions to funding agencies in Europe as well as to NASA.