[...] Damage from naturally strong radiation near Jupiter had left Galileo's tape recorder inoperable for weeks. Galileo's flight team traced the problem to a light-emitting diode in the electronics controlling the motor drive, and then gradually and carefully completed a successful long-distance repair job.
[...] The recovery was achieved by running a current through the damaged diode to anneal, or repair, radiation-caused damage. The first annealing attempt of six hours produced barely discernible improvement. Three additional treatments, for a total of 83 more hours of annealing treatment, produced progressive improvements, to the point that the tape recorder can run for about an hour at a time.
[...] The diode that radiation apparently damaged in the tape recorder is a gallium-arsenide semiconductor component that emits light. The motor-drive control has three of them. Light from them shines through windows in a rotating wheel onto detectors on the other side of the wheel. That setup senses the turning of the wheel and feeds digital logic that controls drive signals for the motor.
The damage apparently came from high-energy protons from Jupiter's radiation belt displacing atoms in the semiconductor's crystalline molecular lattice. Passing a current through the diode for hours serves as a way for electron flow to cause some of the displaced atoms to shift back to their original lattice positions.
Galileo has nearly depleted its supply of the propellant needed for pointing its antenna toward Earth and controlling its flight path. While still controllable, it has been put on a course for impact into Jupiter next September. The maneuver prevents the risk of Galileo drifting to an unwanted impact with the moon Europa, where it has discovered evidence of a subsurface ocean that is of interest as a possible habitat for extraterrestrial life.
Galileo Gets a Long-Distance Repair Job
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