NASA's Galileo spacecraft will reach a milestone Saturday as it performs its 100th scheduled engine burn to fine tune its path around Jupiter.
The sturdy craft has withstood intense radiation at Jupiter to return data on the Jovian system, including its four largest moons, since 1995, and the mission has been extended twice.
The Saturday maneuver, designed to aim Galileo for a flyby of Io in October, could last as long as nine hours. But before the engine burn, engineers will perform a calibration of the craft's gyroscopes used to keep it correctly oriented in space during engine burns and throughout the mission.
Engineers at NASA's Jet Propulsion Laboratory, which manages the Galileo mission, will do some maintenance on Thursday of the probe's on-board tape recorder. At least once every 30 days, the tape is wound at high speed from one end to the other and back again to help reduce the mechanical stickiness which has plagued the operation of the recorder in the past.
Normal playback operations consist of many small low-speed motions back and forth, as small amounts of data are read into the spacecraft computer memory, processed, and packaged for transmission to Earth at rates of 20 to 160 bits per second.
When you compare this to a typical computer modem, which can communicate at 56,000 bits per second, you can begin to see why it takes a month or two for Galileo to completely read out a full tape, which can contain nearly a gigabit (1,000,000,000 bits) of data. At the modem speed, this amount of data could be transmitted in approximately 5 hours.
Earlier this week, the team calibrated the Near Infrared Mapping Spectrometer (NIMS) aboard Galileo by directing its gaze to a target plate on the probe which is heated to a known temperature. Measuring that signal will let scientists accurately gauge the true temperatures of the atmospheric and surface scenes captured by the instrument during the Aug. 5 flyby.
The probe's tape will be played back this week to collect Io flyby data, including observations made by NIMS and others by a Photopolarimeter Radiometer instrument (PPR).
The NIMS observations are of the Gish Bar and Amirani hot spots on Io, measuring temperatures and sulfur dioxide distribution in those areas.
The PPR measurement is a temperature map of Io along a single strip that runs from the north to the south pole, along a line representing roughly noon relative to the 42-hour "day" of the satellite. These observations were taken an hour to an hour and a half after the closest approach to Io on Aug. 5.
On Sunday, engineers will do some remote maintenance work on Galileo's propulsion system and then turn the craft 4 degrees to keep its communications antenna pointed at Earth.
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