He has been a Principal Investigator for Celestial Mechanics (dynamic science) on a number of NASA missions including Mariner 5 to Venus, Mariner 6 and 7 to Mars, Galileo to Jupiter, and Pioneer 10 and 11 to Jupiter, Saturn, and the heliosphere. He has been a member of Radio Science teams on other missions, including Mariner 9 to Mars, Mariner 10 to Venus and Mercury, and Voyager 1 and 2 to Jupiter, Saturn, Uranus, and Neptune. He is currently a Radio Science Team Member on Cassini to Saturn and on ESA's Rosetta comet mission and SMART 1. He has been elected a Fellow of the American Geophysical Union, and he received NASA's exceptional scientific achievement medal for his work on Pioneer 10 and 11.