"Galileo" arrived at the Jupiter system in 1995 and, from 1996 to 2001, regularly monitored volcanic activity on Io through observations of Io's thermal emission at near-infrared wavelengths, imaging Io while it was in the Jupiter's shadow in order to look for thermal hotspots at visible and near-infrared wavelengths, and imaging Io during most orbit in order to detect changes in the appearance of diffuse material and lava flows on the surface. Thermal emission from Pele was detected in nearly every occasion Io's trailing hemisphere was imaged while the moon was in the shadow of Jupiter. The volcanic plume at Pele was found to be intermittent or primarily composed of gas with occasional bursts of increased dust content. It was detected only twice by "Galileo" in December 1996 and December 2000. In these two detections, the plume height varied from to . The plume was also detected by the Hubble Space Telescope in October 1999 while "Galileo" was conducting a flyby of the moon. The Hubble observations allowed for the detection of diatomic sulfur (S2) for the first time on Io in Pele's plume. Subtle changes in the shape and intensity of the large red-ring plume deposit surrounding Pele were observed in daylight images of the volcano, with the most notable change seen in September 1997 when dark pyroclastic material from an eruption of Pillan Patera covered up a portion of Pele's plume deposit.