NASA's Galileo spacecraft soars above Jupiter's volcanic moon Io in this artist's impression. Photo: NASA/JPL

Celebrating the news of official approval of a new mission extension, Galileo continues to return science data acquired during its most recent flyby of Jupiter's volcanic moon Io. During the flyby, the spacecraft flew over the surface of Io at an altitude of 198 kilometers (123 miles), or about the same distance as between Los Angeles and San Diego. This week's data return contains observations made the Photopolarimeter Radiometer (PPR), the Near Infrared Mapping Spectrometer (NIMS), the Solid-State Imaging camera (SSI) and the Fields and Particles instruments. Data playback is interrupted once this week. On Thursday, the spacecraft performs a test to determine the status of the Ultraviolet Spectrometer (UVS) instrument.

UVS has been turned off for Galileo's past three encounters. The instrument's electronics appear to have been damaged by some combination of Jupiter's severe radiation environment and over a decade spent in the harsh space environment. Engineers are hoping that time will allow the damaged electronics to anneal, restoring the instrument to a working state. Annealing is the process in which defects in a material (in this case, radiation damage to a semiconductor) migrate toward the edges of the affected component, thus restoring the material to something nearer to its original, undamaged state.

In this week's playback, the Fields and Particles instruments continue to return parts of an 82-minute high resolution recording of the plasma, dust, and electric and magnetic fields surrounding Io. The data will allow scientists to better understand the interaction between Io, the Io torus, and the Jovian magnetosphere. PPR follows next on the playback schedule with the return of two observations of the Loki volcano. The observations contain temperature measurements that will allow scientists to study the flow of heat on Io's surface.

The Zal Patera region of Jupiters volcanic moon Io is shown in this combination of high-resolution black and white images taken by NASA's Galileo spacecraft on November 25, 1999 and lower resolution color images taken by Galileo on July 3, 1999. Photo: NASA/JPL

Next, NIMS returns an observation of the Pele volcano region designed to map thermal emissions at very high spatial resolutions, allowing dectection of features as small as 1 kilometer (0.6 miles) in size. The observation contains a view of Pele while it is on Io's night side. SSI also returns several nightside images of Pele in an attempt to observe hot, glowing lava in the Pele caldera. PPR follows with the return of a polarimetry observation from the Mulungu Patera region. With the measurements contained in this observation, scientists are hoping to learn more about the particle sizes, textures, and composition of sulfur frost.

SSI returns the remainder of this week's observations. The first is of a feature that appeared to have been affected by "sapping" in an observation that was made in June 1999. Sapping is the natural process of erosion along the base of a cliff by which soft layers are worn away. The erosion removes the support of the upper part of the cliff which then breaks off in large blocks and falls from the cliff face. SSI's next observation contains images of a new hot spot identified during Galileo's November 1999 flyby of Io and designated Chaac Patera. In this very high-resolution observation, surface features as small as 9 meters (30 feet) should be detectable. SSI's final observation is a set of high resolution images of the Prometheus region. At a resolution of 13 meters (43 feet), the observation is expected to capture Prometheus' plume source and active lava flow.