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Io is about the size and density of Earth's Moon, but it has a dramatically different thermal state and geologic history. Tidal heating leads to intense volcanic and tectonic activity, so Io is an accelerated experiment in planetary geology and geophysics. Here we can witness volcanic activity on very large scales and at very high temperatures, perhaps providing insights into the early histories of the terrestrial planets.

Io has always been a high-priority objective of Galileo, but her proximity to Jupiter's intense radiation belt has meant saving the best for last. An Io flyby occurred during Jupiter orbit insertion in December 1995, but a tape recorder anomaly led to cancellation of remote-sensing observations. No further close flybys of Io were planned until the final two orbits (I24 and I25) of the Galileo Europa Mission (GEM) late in 1999 in order to minimize radiation damage to spacecraft and instrument electronics. I24 was an exciting encounter, with a spacecraft safing event and recovery just hours before Io closest approach. Most of the Io data were acquired, but not without problems. The majority of images from the Solid State Imager (SSI) were acquired in a mode that failed due to the cumulative radiation damage, producing garbled images, although reconstruction has produced some useful pictures [Keszthelyi et al., this issue]. Also, the grating of the Near-Infrared Mapping Spectrometer (NIMS) became stuck leading to loss of spectral sampling [Lopes et al., this issue]. These instrument problems were mitigated by the use of different observation modes or strategies in subsequent orbits. The 125 polar pass included another safing event, this time eliminating the recording of the highest-resolution observations. However, the medium-resolution observations were of excellent quality and provided a surprise look at a new fissure eruption at Tvashtar Catena. GEM was followed by the Galileo Millennium Mission (GMM) including another close Io flyby in orbit I27, a fully successful encounter. Three additional Io flybys are anticipated in August 2001 (I31), October 2001 (I32), and January 2002 (I33). The groundtracks for all six Io encounters are shown in Plate 1.