  The Pioneer 10/11 missions, launched in 1972 and 1973, and their navigation
are reviewed. Beginning in about 1980 an unmodeled force of ~ 8 x 10^{-8}
cm/s^2 appeared in the tracking data, it later being verified. The cause
remains unknown, although radiant heat remains a likely origin. A set of
efforts to find the solution are underway: a) analyzing in detail all available
data, b) using data from the New Horizons mission, and c) considering an ESA
dedicated mission.
