Galileo Continues Scientific Observations

Galileo passes the half-way point between encounters as it flies through apojove on Wednesday, November 3, 1999. Apojove is the point at which the spacecraft is furthest from Jupiter in a given orbit. Galileo continues to return science data to Earth, as it starts heading back toward the heart of the Jupiter system and its next, even more daring, flyby of Io. The current set of data now being returned was acquired during Galileo's previous close flyby of Io on October 10 (Pacific Time). Galileo's next encounter with Io will occur on November 26 (Pacific Time).

This week's playback contains observations made by the Near-Infrared Mapping Spectrometer (NIMS), the Solid-State Imaging camera (SSI) and the Fields and Particles (F&P) instruments. The F&P instruments are comprised of the Dust Detector, Energetic Particle Detector, Heavy Ion Counter, Magnetometer, Plasma Detector, and Plasma Wave instrument.

Playback plans contain observations from a third pass through the data stored on Galileo's onboard tape recorder. Additional passes through the tape recorder allow replay of data lost in transmission to Earth, reprocessing of data using different parameters, or return of additional new data. Unfortunately, some of the images being returned by SSI are known to be corrupted. They are being returned with the hope that careful processing will allow some or all of the image data to be recovered.

Data playback is interrupted twice this week to perform engineering and navigation activities. On Tuesday, the spacecraft executes a relatively large flight path adjustment. The adjustment will change Galileo's flight path so the spacecraft flies over Io's south pole during its next encounter. Galileo's previous flyby of Io was near-equatorial. On Thursday, Galileo performs standard maintenance on its onboard tape recorder.

The first two observations returned this week were taken by NIMS and SSI and contain data on the Pele volcanic region. The observations were taken while Pele was on Io's night side. The NIMS data will be used to search for thermal emissions from the Pele caldera, while the SSI observation consists of high-resolution images of the region. The images were taken in the dark with the hope of catching hot glowing lava near Pele's volcanic vent.

Next, SSI returns three observations. The first contains high-resolution images of the Pillan volcanic region. The images were taken at daybreak on Io and provide the best lighting conditions for showing details of the surface topography. High-resolution images of the Colchis Montes region are returned in SSI's next observation, followed by high-resolution images of the Zamama volcanic vent. NIMS also returns an observation of Zamama, designed to provide scientists with information describing the surface composition of the region.

The F&P instruments then take the stage and return data from a 65-minute high-resolution recording of the environment (plasma, dust, and electric and magnetic fields) surrounding Io. These data will assist scientists with studies of the Io ionosphere and its interaction with the Jovian magnetosphere.

SSI returns to the playback schedule with the return of images of the Prometheus volcanic vent and associated lava flows. A comparison of clear and green filter images of this region are expected to reveal unresolved lava and allow scientists to determine surface temperatures of the area.

NIMS returns to the playback schedule with the return of an observation of the Colchis Montes region. SSI also returns images of Colchis, providing a wider, lower resolution view of the region and context for the higher resolution images returned earlier in the week. Next on the schedule is the return of SSI and NIMS observations of Tohil Mons. The Tohil and Colchis features are mountains, whose geological structure, origin and history are presently unknown.

These observations are followed by a return of another couple of observations of Prometheus; one for each of SSI and NIMS. The SSI observation will provide color images, which will also be combined with the previous set of images to yield stereo coverage. Finally, SSI returns another observation of the Zamama volcanic vent, again providing coverage of a wider region as context for the higher resolution images returned earlier.