Galileo will travel by Venus, through the asteroid belt and fly twice past Earth and its moon. The Ulysses mission to the sun, jointly sponsored by the European Space Agency (ESA) and NASA, will involve the first spacecraft to explore the sun's polar regions by using a gravity assist from Jupiter to travel out of the solar system's ecliptic plane. It will be launched in October 1990. Ulysses will give us a unique, uninterrupted view of the sun's polar regions, enabling us to observe firsthand the solar corona, the solar wind, the heliosphere's magnetic field, both solar and non-solar cosmic rays, and interstellar and interplanetary neutral gas and dust. Both of these missions have had long development histories due to the US Shuttle's extended development time and the Challenger accident. During their flights, Galileo and Ulysses will be powered by electricity from radioisotope thermoelectric generators (RTGs). Earlier versions of these generators were used on all previous deep-space probes (Pioneer, Viking, Mars Lander and Voyager) as well as the Apollo missions. These devices have also been used since the 1960s in navigation and weather satellites. Use of radioisotope thermoelectric generators for spacecraft power and radioisotopic heater units (RHUs) for heating present unique safety analyses requirements for launch approval. These safety analyses have been re-done for Galileo since the Challenger accident, and they will be addressed for Ulysses in the near future. This paper presents a brief review of the Galileo and Ulysses mission objectives and design approaches. The RTG design, safety analysis process and safety test results are presented. Finally, a brief summary of Galileo status as it nears the launch period is presented. Galileo Mission The Galileo Mission was approved in 1978 to carry out detailed exploration of the Jovian System as a follow-up to the Pioneer and Voyager missions of the 1970s. The Galileo Mission objectives are to: • investigate the chemical composition and physical state of Jupiter's atmosphere; • investigate the chemical composition and physical state of the Jovian satellites: Io, Europa, Ganymede and Callisto; and • investigate the structure and physical dynamics of the Jovian magnetosphere. These mission objectives will be accomplished by a multipurpose spacecraft (Fig. 1). The major elements are: • an orbiter to tour and study the Jovian satellites over a 20-month period, and • a detachable probe to make a one-hour descent through the Jovian atmosphere and relay measurements, via the Orbiter, to Earth. The major design drivers for the Galileo spacecraft and mission are illustrated in Fig. 2. The competing requirements of fields and particles and imaging instruments dictated a dual-spin spacecraft design. The combination of the orbiter and its propulsion system resulted in a spacecraft mass of 2670 kg. The combined power requirements of long life, ability to operate in the low solar intensity at Jupiter and ability to operate in Jupiter's intense radiation environments led to selection of RTGs for spacecraft power similar to those used on the Pioneer and Voyager spacecraft. The third mission design driver relates to the mission energy requirements. Prior to 1986, the Galileo mission was to be launched on a direct trajectory to Jupiter with the Shuttle/Centaur launch vehicle. However, the Centaur was cancelled for Shuttle use
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