  The radio-metric Doppler tracking data from the Pioneer 10/11 spacecraft,
from between 20-70 AU, yields an unambiguous and independently confirmed
anomalous blue shift drift of a_t = (2.92 \pm 0.44)\times 10^{-18} s/s^2. It
can be interpreted as being due to a constant acceleration of a_P = (8.74 \pm
1.33) \times 10^{-8} cm/s^2 directed towards the Sun. No systematic effect has
been able to explain the anomaly, even though such an origin is an obvious
candidate. We discuss what has been learned (and what might still be learned)
from the data about the anomaly, its origin, and the mission design
characteristics that would be needed to test it. Future mission options are
proposed.
