The Pioneer anomaly entered the phenomenology of modern physics at a time when researchers have mounted significant efforts to investigate two other observational anomalies: the flat rotation curves of spiral galaxies and the accelerated expansion of the universe. Although the true origin of the anomalous acceleration of the Pioneer 10 and 11 spacecraft is yet to be reported, its existence has already helped us deepen our understanding of gravity. Unlike the other two anomalies, the Pioneer anomaly is “local” in character, taking place within our own solar system, involving a spacecraft of our own making. As a result, the Pioneer anomaly has led to increasing interest in space-based fundamental physics research focusing on solar system tests of gravity. This renewed interest is, perhaps, the most significant contribution of the Pioneer effect to modern physics to date.
The presence of the unexpected acceleration signal in the Pioneer radiometric Doppler data has led to a re-evaluation of theoretical frameworks used to test gravity in the solar system. It has also motivated the development of new physical mechanisms to explain the effect. The anomaly has necessitated a re-analysis of many physical concepts, resulting in studies of the behavior of gravitationally bound systems in an expanding universe, investigations of gravity modification mechanisms, and efforts to detect the anomaly with other spacecraft, planets, and other bodies in the solar system. The anomaly has also motivated the development of new methods to improve the accuracy of spacecraft navigation, including methods of accounting for the effect of thermal recoil forces.
In this review we describe the Pioneer 10 and 11 spacecraft. We provide a significant amount of information on the design, operations and behavior of the two Pioneers during their entire missions. This includes information from original project documentation and descriptions of various data formats and techniques that were used for acquisition of the Pioneer data. We describe the radiometric Doppler data and techniques for data preparation and analysis. We also discussed the Pioneer telemetry data and its value for the anomaly investigation. We review the observational techniques and physical models that were used for precision tracking of the Pioneer spacecraft. We summarize the current knowledge of the physical properties of the Pioneer anomaly and review various mechanisms proposed for its explanation.
October 5, 2009 marked the eleventh anniversary of the first announcement of the Pioneer anomaly  (see also [27, 390]). In the decade that followed, the existence of the anomalous acceleration in the radiometric Doppler data received from the Pioneer 10 and 11 spacecraft was confirmed by several independent researchers [179, 194, 274, 377]. Thus, the existence and approximate magnitude of the anomaly can be considered established fact. However, the direction of the acceleration remains unclear: the four principal directions (sunward, earthward, along the spin axis, or along the velocity vector) fall within a few degrees of each other and based on available data, cannot be distinguished easily. The temporal behavior of the anomaly has been put into question by studies [194, 377] that demonstrate the presence of a jerk term. Meanwhile, other studies (see [171, 172, 400, 328, 329, 376], followed by [51, 311, 312]), indicate that the magnitude of acceleration due to thermal recoil forces of on-board origin may be significantly larger than previously estimated. As a result, the question of the origin of the Pioneer anomaly remains open, but hopefully not for long.
A comprehensive investigation of the anomaly has recently begun. The new study relies on the much-extended set of radiometric Doppler data for both spacecraft in conjunction with the entire record of the Pioneer 10 and 11 spacecraft telemetry files and large archive of project documentation. This unique information has already led to the development of new software tools capturing all aspects of the in-flight behavior of the Pioneer vehicles throughout their missions. These efforts may soon reveal the origin of the anomaly. Our review provides the necessary background for the new results to appear in the near future. Such an anticipated development makes the study of the Pioneer anomaly a good subject for Living Reviews.
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