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This Week on Galileo?
October 11, 1999

Galileo Continues Observation Campaign of Volcanic Moon

A few short hours after passing within a few hundred kilometers of Io's surface, Galileo continues its observation campaign of the volcanic moon. Early on Sunday morning, the spacecraft went into safe mode when a memory error was detected in one of the onboard flight computers, while the spacecraft began the first of the planned recordings. The memory error was apparently caused by the intense radiation that the spacecraft is exposed to during the Io encounter. Through an intense and heroic effort, the flight team was able to quickly recover the spacecraft and Galileo resumed its flight sequence about 2 hours prior to the Io flyby.

Today's activities include observations performed primarily by the Remote Sensing instruments, but the Fields and Particles instruments also continue their low resolution survey of the Jovian magnetosphere. Galileo's observations of Io are completed today, as is the magnetosphere survey. Playback of the data stored on the spacecraft's onboard tape recorder during the last couple of days is initiated at 11:00 pm PDT-SCET (11:33 pm PDT-ERT, see Note 1).

The Near-Infrared Mapping Spectrometer (NIMS) makes the first observation of the day. In it, the NIMS looks at the plume of the Pele volcano. The geometry of the observation is such that the plume, if present, will be on Io's limb, with Jupiter's disk in the background.

After NIMS completes its observation, the Solid-State Imaging camera (SSI) performs a regional observation of Io. Images taken during this observation will be combined with images taken in July to produce stereo views of the region. The NIMS also takes a regional look at Io, providing data that will be used to study surface composition and thermal emissions. Next, the SSI captures 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.

The Extreme Ultraviolet Spectrometer (EUV) enters the encounter scene with a distant look at Io's torus. The Io torus is a doughnut-shaped region with its inner edge bounded by Io's orbit. It is a region of particularly intense plasma flow and radiation activity inside the Jovian magnetosphere. The Io torus is constantly replenished by volcanic activity on Io. Similar observations have been performed during Galileo's previous encounters, and the aggregate data set will allow scientists to examine long term variations in the torus' size and shape.

The SSI then looks at Io while it is eclipsed from the Sun by Jupiter. SSI captures a color view of Io, with the volcanic regions Loki, Pele, Pillan and Marduk on the moon's limb. Being on the limb will facilitate the identification of plumes that may be present. In addition, comparison of clear filter and 1-micron images will enable measurement of hot spot temperatures. The other color images will aid in identifying the nature and source of diffuse atmospheric emissions.

The encounter's observation schedule is closed out today with a final remote look at the Io torus. Again, the EUV adds more data to the existing set that will allow scientists to perform long term morphological studies.

Come back tomorrow and learn what data will be transmitted to Earth during the remainder of the week. Look for the return of This Week on Galileo!

Note 1. Pacific Daylight Time (PDT) is 7 hours behind Greenwich Meridian Time (GMT). The time when an event occurs at the spacecraft is known as Spacecraft Event Time (SCET). The time at which radio signals reach Earth indicating that an event has occured is known as Earth Received Time (ERT). Currently, it takes Galileo's radio signals 33 minutes to travel between the spacecraft and Earth.

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