junction of about 300°C to produce a thermoelectric efficiency of about 9%. Waste heat is radiated from the finned RTG housing which is covered with a high emissivity coating. The overall dimensions of the GPHS-RTG are 42.2 cm diameter (fin tip to fin tip) by 114 cm long. It weighs 55.8 kg for a specific power (at BOM) of 5.1 We/kg. The GPHS-RTG is equipped with cooling passages through its fin roots so that an auxiliary pumped-liquid cooling system can remove its heat while installed in the shuttle payload bay. The auxiliary cooling system is used prior to, and during, launch until the shuttle payload doors are opened on orbit and the spacecraft is deployed or during return to earth, if necessary. Two GPHS-RTGs were scheduled for launch on the Galileo mission in October. Another GPHS-RTG is ready for launch on the Ulysses mission one year later. A fueled spare GPHS-RTG is also available as a backup for these missions. A fueled GPHS-RTG qualification test unit has been on life test under simulated space thermal/vacuum conditions since 1984. Due to a change (since the Challenger accident) from a Centaur upper stage to the less energetic Interim Upper Stage (IUS) vehicle, these missions will employ a gravity-assist trajectory around Venus and twice around the Earth which will increase trip time to reach their destinations. For example, it will take six years for the Galileo spacecraft to reach Jupiter. The long-lived capability of the GPHS-RTG will easily meet this somewhat greater challenge to add to our knowledge of the solar system. CRAF/Cassini The next step along the way to learning about our solar system and how it was formed which will require radioisotope power systems is NASA’s Mariner Mark II spacecraft program. The first segment of this program includes the launch of the Comet Rendezvous Asteroid Flyby (CRAF) mission scheduled for August 1995 and a similar spacecraft launch on the Cassini mission in April 1996. CRAF is planned to flyby one or more asteroids on its way to rendezvous with a comet and deliver a penetrator into the nucleus of the comet. Cassini is scheduled to orbit Saturn and its moon Titan, and to deliver a scientific probe into Titan’s atmosphere. The CRAF mission will last over eight years and the Cassini mission will end in the year 2006, over 10 years after launch. Both spacecraft are scheduled to be launched on Titan IV/Centaur vehicles. Each spacecraft will require 560 to 600 We at time of launch. The baseline power system option for the CRAF/Cassini missions is to use two GPHS-RTGs per spacecraft just like those planned for use on Galileo and Ulysses. The primary reasons for this selection are program cost and schedule considerations, especially having to do with the availability and upgrading of facilities for fuel production, processing and encapsulation. Further, the GPHS-RTG unicouple technology is proven for such long-lived missions and two spare GPHS-RTGs will be available from the upcoming launches. The CRAF mission, the earliest of the two, requires less power and does not last as long as the Cassini mission. Therefore, the flight spare GPHS-RTG plus an unfueled GPHS-RTG from the Galileo/Ulysses program are targeted for use on the CRAF spacecraft in 1995. Since the flight spare RTG will have been fueled for about 10 years by the time the CRAF mission is to be launched, its power output will be lower (due to decay of the Pu-238 fuel) and its high energy gamma radiation levels (from the buildup of Pu-236 daughter products) will be higher. We are currently studying whether or not this flight spare RTG will have to be defueled and refueled with a fresh
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