Ulysses (spacecraft)

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Ulysses was a robotic space probe designed to study the Sun as a joint venture of NASA and the European Space Agency (ESA). The spacecraft was originally named Odysseus, because of its lengthy and indirect trajectory to near Solar distance. It was renamed Ulysses, the Latin translation of "Odysseus" at ESA's request in honour not only of Homer's mythological hero but also with reference to Dante's description in Dante's Inferno,[1] Originally scheduled for launch in 1986 aboard the Space Shuttle Challenger, due to the loss of Challenger, the launch of Ulysses was delayed until October 6, 1990 aboard the Space Shuttle Discovery (mission STS-41). The spacecraft's mission was to study the Sun at all latitudes. To do this required a major orbital plane shift. Due to velocity change limitations of the Shuttle and the Inertial Upper Stage (IUS), this was accomplished by using an encounter with Jupiter to effect the plane change instead of an engine burn. The need for a Jupiter encounter meant that Ulysses could not be powered by Solar cells and was powered by a radioisotope thermoelectric generator (RTG) instead.

By February 2008, the power output from the RTG, which is generated by heat from the radioactive decay of plutonium-238, had decreased enough to leave insufficient power for internal heaters to keep the spacecraft's attitude control hydrazine fuel from freezing. The end of mission was at one point scheduled for July 1, 2008, but mission scientists came up with a method to keep the fuel liquid by conducting a short thruster burn every two hours, allowing the mission to continue.[2][3][4] The cessation of mission operations and deactivation or hibernation of the spacecraft was determined by the inability to prevent attitude control fuel from freezing.[2][5] The last day for mission operations on Ulysses was 30 June 2009.[6][7] This was a full year after the most recent previously announced mission end date. The scheduled end of mission in 2009 was the fourth time that the end of the spacecraft's mission had been scheduled.[2] The last scheduled ground station pass of the mission was over the Madrid Deep Space Network 70m ground station (DSS-63) from around 15:35 to 20:20 UTC. There were no decommissioning engineering tests on the spacecraft.

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