He is known for his investigations of the Pioneer Anomaly, affecting Pioneer 10 and Pioneer 11 spacecraft, and for his attempt to recover early data of the Pioneer spacecraft to shed light on such a phenomenon.
Spacecraft can be used to observe the solar eclipses on Jupiter, these include Pioneer 10 and Pioneer 11 (1973 and 1974), Voyager 1 and Voyager 2 (1979), Galileo orbiter (1995-2003), Cassini-Huygens (2000) and New Horizons (2007) observed the transits of their moons and its shadows.
He was Principal Investigator for the Imaging Photopolarimeter experiment on the Pioneer 10 and Pioneer 11 first flybys of Jupiter and Saturn in the 1970s.
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According to Slava Turyshev of JPL in the paper "Support for temporally varying behavior of the Pioneer anomaly from the extended Pioneer 10 and 11 data sets," published in Physical Review Letters in 2011, the anomaly has a temporally-decaying (not constant as previously thought) nature and points towards Earth.