With this issue, we begin answering readers' questions about the Galileo mission. We received quite a few in last month's readers' survey and others over the Internet. We will try to answer at least one question in each issue.

Q:

Are there any plans to observe Jupiter's minor satellites? If so, what type?

A:

The Imaging Team has developed a comprehensive plan for observing Jupiter's four small inner satellites--Amalthea, Thebe, Adrastea, and Metis. The plan involves imaging each satellite at each of nine geometries. These geometries consist of three illumination conditions at each of three longitudes spaced approximately 120 degrees apart. The illumination conditions are:

• Low solar phase angle (Sun-satellite-spacecraft angle less than 20 degrees)
• Moderate solar phase angle (40-65 degrees) with the Sun from the east
• Moderate solar phase angle with the Sun from the west

These nine geometries will allow us to study the size, shape, spin state, colors, and photometric properties of these small satellites, as well as ephemeris improvements for Adrastea and Metis and improved understanding of surface morphology on Amalthea and Thebe. The low-phase observations are accomplished using four-color on-chip mosaics recorded with SSI internal camera data compression; the moderate-phase observations are done in a single color and will be windowed and compressed by the spacecraft computer. Most of the low-phase observations are scheduled for Galileo's ninth orbit around Jupiter, when there is ample downlink capability. Use of on-chip mosaicking requires that the range from Jupiter be >10.8 Jupiter radii to keep radiation noise acceptably small. Resolutions are typically 5-10 km/pixel providing 3 to 40 pixels across the bodies, depending on the observation and the target body.

Because the ranges to the outer small satellites from Galileo are so large, ground-based observations are just as good with much higher power telescopes.