On Pioneer 11, one Geiger-Müller tube was replaced by a thin silicon wafer to detect protons in the specific energy range 0.61 to 3.41 MeV.
Acuña was a principal investigator on magnetometer experiments flown on numerous missions over the years, from the Pioneer 11 Fluxgate Magnetometer Experiment in 1973 to the Mars Global Surveyor Magnetic Field Experiment in 1994, and the MESSENGER magnetometer.
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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