Galileo Continues Return of Science Data

Galileo's primary activity this week is the continued return of science data acquired during the spacecraft's close flyby of Io earlier this month. Observations made during the flyby are safely stored on Galileo's onboard tape recorder. This week's data is returned from observations made by the Near-Infrared Mapping Spectrometer (NIMS) and Solid-State Imaging camera (SSI). Data playback is interrupted twice this week. On Tuesday, the spacecraft performs a small turn to keep its radio antenna pointed to Earth. On Wednesday, Galileo performs a standard gyroscope performance test.

First on the playback schedule is the return of an observation taken by SSI. The observation contains images of a region of Io's surface near the terminator (or line dividing night from day). The oblique lighting provides conditions that are optimal for studying the topography of a region containing the Hi'iaka caldera. Next, NIMS returns a regional observation of Io designed to study surface composition and detect thermal emissions.

The next pair of observations are returned by SSI and NIMS, and focus on the Pillan plume. The geometry of both observations is such that the Pillan hot spot was situated on Io's limb as seen from the spacecraft. If Pillan was active during the observations, its plume will be seen against the dark sky above the limb. The data will provide scientists with the best look to date at a plume's size, shape, and composition. NIMS returns a similar observation that may contain a plume of the Pele volcano. This observation's geometry is such that the plume, if present, will be seen with Jupiter's disk in the background.

The SSI camera then returns a regional observation of Io. The images taken during this observation will be combined with images taken in July to produce stereo views of the region. Following on the schedule is the return of a second regional scan of Io performed by NIMS. SSI then returns an observation that contains a color view of Io's full disk, including the best color coverage of the region of Io containing the Loki and Pillan volcanoes, with the Acala region on Io's limb. SSI also returns an observation of Io while it was eclipsed from the Sun by Jupiter. In this color image, the volcanic regions of Loki, Pele, Pillan and Marduk are on Io's limb. This geometry will facilitate the identification of plumes that may be present. In addition, comparison of images taken through different color filters will enable scientists to measure hot spot temperatures and possibly identify the nature and source of diffuse atmospheric emissions.

Toward the end of the week, Galileo starts another pass through the observations stored on the tape recorder. This pass allows replay of data lost in transmission to Earth, reprocessing of data using different parameters, or return of additional new data. Two observations are returned by SSI this week. The first observation contains high-resolution images of the Pele region. The images were taken with Pele in darkness with the hope of catching hot glowing lava near Pele's volcanic vent. The next observation consists of high-resolution images of the Pillan volcanic region, taken at daybreak on Io with oblique viewing geometry.