expensive spacecraft were flown (Figure 7.29). The spacecraft weigh 270 kg each and the diameter of the main radio dish is 2.7 m. The spacecraft are spin-stabilized, spinning at 5 rpm, and are powered by four radioisotope thermoelectric generators (RTGs), each of which provided 40 W power at launch.

Pioneer 10 was launched on March 2, 1972 and flew past Jupiter on December 3, 1973 at an altitude of only 1.82 Rj (130,354 km). It was then directed to the outer reaches of the solar system and is now heading in the direction of the star Aldebaran, as can be seen in Figure 7.30. Pioneer 10 is still in radio contact and it may have been deflected in 1999 by an encounter with a Kuiper Belt Object. In addition, detailed tracking of Pioneer 10 spacecraft (and other spacecraft now at great distance from the Sun) has revealed a tiny unexplained acceleration towards the Sun (Anderson et al., 1998). The nature of this mysterious anomaly has so far defied analysis and may even indicate a small discrepancy in gravitational theory itself! As the mission has proceeded, the power generated by the RTGs has steadily fallen, partially because of the 92-year half-life of the Plutonium-238 isotope used, but mainly due to degradation of the thermocouple junctions that convert the heat to electricity. Pioneer 10 is expected

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