However, the propellant supply needed for steering the spacecraft and keeping its antenna pointed toward Earth is now nearly exhausted. The tour has relied on expert navigators to calculate several moves in advance, using each moon's gravity to help adjust the spacecraft's trajectory toward its various encounters. In its 33 loops around Jupiter, it has flown near Io six times previously and near the other three of Jupiter's planet-sized moons - Europa, Ganymede and Callisto - a total of 27 times. The resilient spacecraft has survived about three and a half times as much exposure to radiation from Jupiter's radiation belts as it was designed to withstand. Galileo has operated in orbit more than three times longer than its originally planned mission. "The reason we're going so close is to put Galileo on a ballistic trajectory for impact into Jupiter in September 2003," Theilig said. The Galileo flight team at JPL aimed the orbiter to skim just 100 kilometers (62 miles) above Io's multicolored surface at 14:09 Universal Time (6:09 a.m. That's been great for studying Io, since it keeps changing so much." "An orbital mission like Galileo gives you the advantage of getting to examine interesting places repeatedly over a period of time. Eilene Theilig, Galileo project manager at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "Galileo's days are numbered now, so it's especially exciting to visit Io one last time," said Dr. Scientists hope this week's encounter will reveal how several regions of Io have changed over the years. The rotor had one 4.8 meter high-gain antenna and two low-gain antennas, but the high-gain antenna did not deploy so data were returned from Jupiter at rates far below the design maxima using the low-gain antennas.Io's volcanoes have presented many surprises since they were first seen in 1979 by NASA's Voyager spacecraft and especially during the six years that Galileo has been orbiting Jupiter. The telecommunications subsystem was designed to transmit data to Earth at rates ranging from 10 bps to a maximum of 134 kilobits per second at S-band and X-band frequencies. The command and data subsystem consisted of multiple microprocessors and a high-speed data bus. Propulsion was provided by a bipropellant system of twelve 10-newton thrusters and one 400 newton engine. The spacecraft power source was a pair of radioisotope thermoelectric generators. There were eleven subsystems and nine scientific instruments on the orbiter. The rotor and stator were connected by a spin bearing assembly, which conducted power via slip rings and data signals via rotary transformers. This design accommodated the different requirements of remote sensing instruments (mounted on the stator) and fields and particles instruments (mounted on the rotor) spacecraft engineering subsystems were also mounted on the rotor. The rotor (or spun section) turned at approximately three revolutions per minute while the stator (or despun section) maintained a fixed orientation in space. Galileo used a dual-spin attitude stabilization system.
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