  The Pioneer 10/11 spacecraft yielded the most precise navigation in deep
space to date. However, their radio-metric tracking data received from the
distances between 20--70 astronomical units from the Sun has consistently
indicated the presence of a small, anomalous, Doppler frequency drift. The
drift is a blue frequency shift that can be interpreted as a sunward
acceleration of a_P = (8.74 +/- 1.33) x 10^{-10} m/s^2 for each particular
spacecraft. This signal has become known as the Pioneer anomaly; the nature of
this anomaly remains unexplained.
  Recently new Pioneer 10 and 11 radio-metric Doppler and flight telemetry data
became available. The newly available Doppler data set is significantly
enlarged when compared to the data used in previous investigations and is
expected to be the primary source for the investigation of the anomaly. In
addition, the flight telemetry files, original project documentation, and newly
developed software tools are now used to reconstruct the engineering history of
both spacecraft. With the help of this information, a thermal model of the
Pioneer vehicles is being developed to study possible contribution of thermal
recoil force acting on the two spacecraft. The ultimate goal of these physics
engineering efforts is to evaluate the effect of on-board systems on the
spacecrafts' trajectories.
