The 1972 and 1973 launched Pioneer 10 and 11 were the first missions to explorethe outer solar system. They achieved stunning breakthroughs in deep-space ex-ploration. But around 1980 an unmodeled force of ~ 8 x 10~(-8)cm/s~2, directed approximately towards the Sun, appeared in the tracking data. It later was unam-biguously verified as not being an artifact. The origin remains unknown (althoughradiant heat remains a likely cause). Increasing effort has gone into understandingthis anomaly. We review the situation and describe programs to resolve the issue.
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