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5 Spacecraft malfunctions
5.1 Main antenna failure
For reasons which are not currently known, and in all likelihood will never be known with certainty, Galileos High Gain Antenna failed to fully deploy after its first flyby of Earth. Investigators speculate that during the time that Galileo spent in storage after the Challenger disaster lubricants evaporated, or the system was otherwise damaged. Engineers tried thermal cycling the antenna, rotating the spacecraft up to its maximum spin rate of 10.5 rpm, and "hammering" the antenna deployment motors - turning them on and off repeatedly - over 13,000 times; all attempts failed to open the high gain antenna. Fortunately Galileo had an additional Low Gain Antenna that was capable of transmitting information back to Earth, though since it transmitted a signal isotropically the Low Gain Antenna's bandwidth was significantly less than the high gain antenna's would have been; the high gain antenna was to have transmitted at 134 kilobits per second whereas the low gain antenna was only intended to transmit at about 8 to 16 bits per second. Galileo's low gain antenna transmitted with a power of about 15 to 20 watts, which, by the time it reached earth, and had been collected by one of the large aperture (70 m) DSN antennas, had a total power of about -170 dBm or 10 zeptowatts (10 × 10−21 watts).[21] Through implementation of sophisticated data compression techniques, arraying of several Deep Space Network antennas and sensitivity upgrades of receivers used to listen to Galileo's signal, data throughput was increased to a maximum of 160 bits per second. The data collected on Jupiter and its moons was stored in the on board tape recorder, and transmitted back to Earth during the long apogee portion of the probe's orbit using the low gain antenna. At the same time, measurements were made of Jupiter's magnetosphere and transmitted back to Earth. The reduction in available bandwidth reduced the total amount of data transmitted throughout the mission to about 30 gigabytes and reduced the number of pictures that were transmitted significantly; in all, only 14,000 images were returned.
5.2 Tape recorder anomalies and remote repair
Since Galileo's high gain antenna failed to open in 1991 the mission was forced to use the low gain antenna for all communication to earth. This meant that data storage to Galileo's tape recorder for later compression and playback was absolutely crucial in order to obtain any substantial information from the planned Jupiter and moon flybys. In October of 1995 Galileo's 109 Megabyte tape recorder, after recording an image of Jupiter, remained stuck in rewind mode for 15 hours before engineers learned what happened and sent commands to shut it off. Though the recorder itself was still in working order the malfunction possibly damaged a length of tape at the end of the reel. This section of tape was subsequently declared "off limits" to any future data recording and was covered with 25 more turns of tape to secure the section and reduce any further stresses, which could tear it. Because it happened only weeks before Jupiter Orbit Insertion, the anomaly prompted engineers to sacrifice data acquisition of almost all of the Io and Europa observations during Jupiter Orbit Insertion in order to focus solely on recording data sent from the Jupiter probe descent.
In November of 2002, after completion of the mission's only encounter of Jupiter's moon Amalthea, problems with playback of the tape recorder would again plague the spacecraft. About 10 minutes after closest approach of the flyby Galileo stopped collecting data, shut down all of its instruments, and went into "safe mode"; apparently as a result of exposure to Jupiter's extremely high radiation environment. Though most of the Amalthea data was already written to tape, it was found that the recorder refused to respond to commands telling it to play back data. Through careful analysis [22] after weeks of troubleshooting of an identical flight spare of the recorder on the ground, it was determined that the cause of the malfunction was a reduction of light output in 3 infrared Optek OP133 [23] light emitting diodes located in the drive electronics of the recorder's motor encoder wheel. The GaAs LEDs had been particularly sensitive to proton irradiation induced atomic lattice displacement defects, which greatly decreased their effective light output and caused the drive motor's electronics to falsely believe the motor encoder wheel was incorrectly positioned. Galileo's flight team then began a series of " annealing" sessions, where current was passed through the LEDs for hours at a time to heat them to a point where some of the crystalline lattice defects would be shifted back into place, thus increasing the LED's light output. After about 100 hours of annealing and playback cycles, the recorder was able to operate for up to an hour at a time. After many subsequent playback and cooling cycles, the complete transmission back to earth of all recorded Amalthea flyby data was successful.
