  The radio-metric tracking data received from the Pioneer 10 and 11 spacecraft
from the distances between 20-70 astronomical units from the Sun has
consistently indicated the presence of a small, anomalous, blue-shifted Doppler
frequency drift that limited the accuracy of the orbit reconstruction for these
vehicles. This drift was interpreted as a sunward acceleration of a_P =
(8.74+/-1.33)x10^{-10} m/s^2 for each particular spacecraft. This signal has
become known as the Pioneer anomaly; the nature of this anomaly is still being
investigated.
  Recently new Pioneer 10 and 11 radio-metric Doppler and flight telemetry data
became available. The newly available Doppler data set is much larger when
compared to the data used in previous investigations and is the primary source
for new 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 spacecraft. With the help of this
information, a thermal model of the Pioneers was developed to study possible
contribution of thermal recoil force acting on the spacecraft. The goal of the
ongoing efforts is to evaluate the effect of on-board systems on the
spacecrafts' trajectories and possibly identify the nature of this anomaly.
  Techniques developed for the investigation of the Pioneer anomaly are
applicable to the New Horizons mission. Analysis shows that anisotropic thermal
radiation from on-board sources will accelerate this spacecraft by ~41 x
10^{-10} m/s^2. We discuss the lessons learned from the study of the Pioneer
anomaly for the New Horizons spacecraft.
