third stage. The small spacecraft mass, compared to Galileo, enabled the IUS/PAM-S to capture the original direct trajectory to Jupiter with only some reduction in the prime observation time. Figure 5 illustrates the Ulysses mission profile from Earth in October 1990, with a northern passage over Jupiter in February 1992 and an 86° inclination return to the sun with a closest approach of 1.2 Aeronautical Units (AU) in March 1995. The spacecraft range during the high solar latitude passes will be 2.2 AU, enabling near observation of the poles of the Sun. This trajectory will allow a total of 250 days of observation above a solar latitude of 70° over both poles of the sun. This is the prime observational phase of the mission. The Ulysses spacecraft was returned to ESA for storage in 1986. Work since then has concentrated on replacement of some electronic parts, resetting the thermal radiators to compensate for the reduction in RTG power during the mission lifetime, IUS/PAM design integration and mission design revisions. The spacecraft is currently undergoing instrument integration and system testing in preparation for its October 1990 launch. Radioisotope Thermoelectric Generator (RTG) and Radioisotope Heater Unit (RHU) Descriptions RTG History The RTGs used to power the Galileo and Ulysses spacecraft were derived from a lineage of RTGs used to power US space missions since 1961. These include: • eight missions in Earth orbit (11 RTGs); • five missions to the lunar surface (5 RTGs); and • six missions in planetary exploration (18 RTGs). The RTGs used for these missions have uniformly exceeded lifetime and performance requirements. Several RTG-powered spacecraft are still operational, including Pioneer (17 years), LES 8/9 (13 years) and Voyager (11 years). Figs 6 and 7 illustrate the evolution of RTG power levels and mission length requirements since 1960. Included are missions which have flown, Galileo and Ulysses
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