Radio-metric Doppler tracking data received from the Pioneer 10 and 11 spacecraft
from heliocentric distances of 20 – 70 AU has consistently indicated the presence of a small,
anomalous, blue-shifted frequency drift uniformly changing with a rate of 6 × 10–9 Hz/s.
Ultimately, the drift was interpreted as a constant sunward deceleration of each particular
spacecraft at the level of aP = (8.74 ± 1.33) × 10–10 m/s2. This apparent violation of the
Newton’s gravitational inverse-square law has become known as the Pioneer anomaly; the nature
of this anomaly remains unexplained. In this review, we summarize the current knowledge
of the physical properties of the anomaly and the conditions that led to its detection and
characterization. We review various mechanisms proposed to explain the anomaly and discuss
the current state of efforts to determine its nature. A comprehensive new investigation of
the anomalous behavior of the two Pioneers has begun recently. The new efforts rely on the
much-extended set of radio-metric Doppler data for both spacecraft in conjunction with the
newly available complete record of their telemetry files and a large archive of original project
documentation. As the new study is yet to report its findings, this review provides the necessary
background for the new results to appear in the near future. In particular, we provide a
significant amount of information on the design, operations and behavior of the two Pioneers
during their entire missions, including descriptions of various data formats and techniques used
for their navigation and radio-science data analysis. As most of this information was recovered
relatively recently, it was not used in the previous studies of the Pioneer anomaly, but it is
critical for the new investigation.
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