An artist's concept of NASA's Galileo spacecraft in orbit around Jupiter. Photo: NASA/JPL

Galileo continues to play back science data stored on its onboard tape recorder as the spacecraft flies through apojove and starts its return to the heart of the Jupiter system. Apojove occurs on Thursday and is the point at which the spacecraft is farthest from Jupiter in a given orbit. The current orbit's apojove distance is 154 Jupiter radii (11 million kilometers, or 6.9 million miles). Data playback is interrupted twice this week. On Friday, the spacecraft executes a small flight path adjustment. Late Sunday night, Galileo performs standard maintenance on its propulsion system. The maintenance is performed regularly to prevent debris from accumulating in Galileo's propellant lines and blocking them.

The Near-Infrared Mapping Spectrometer (NIMS) is first to return data this week. A continuation of data playback from last week, the data contain a regional scan of Io's surface. The scan will provide context information for other high-resolution observations. The Photopolarimeter Radiometer (PPR) returns the next observation, which consists of a view of the Shakuru region of Io to determine temperatures of sulfur frost. The Solid-State Imaging camera (SSI) in next on the data return schedule with the playback of a global image of Io.

Toward the end of the week, Galileo begins returning data from a second pass through the observations stored on the tape recorder. This additional pass is primarily for the return of additional data, but it also allows replay of data lost in transmission to Earth, or reprocessing of data using different parameters.

To start off this second pass, PPR returns an observation of Jupiter's atmosphere designed to allow scientists to learn more about the vertical cloud structure of Jupiter, including cloud particle shape and size. The Fields and Particles instruments are next on the playback schedule with the return of portions of a 2-3/4 hour high resolution recording of the Io torus. The torus is a ring-shaped region of intense plasma and radiation activity which is actively maintained by Jupiter's strong magnetic field and Io's constant supply of volcanic particles. The Fields and Particles instruments also return portions of a high resolution recording made during the closest 82 minutes of Galileo's February flyby of Io.

NIMS and SSI take to the playback stage next with the return of observations of Io's Pele volcano. The observations were performed with Pele on Io's night side. The NIMS data are designed to provide a map of thermal emissions surrounding Pele's caldera. SSI contributes by returning several images of the hot glowing lava in the caldera. SSI also returns this week's final observation. The observation contains a feature that was seen to have been affected by sapping in an observation 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 for the upper part of the cliff which then breaks off in large blocks and falls from the cliff face.