7 Current Status of the Study of the Pioneer Anomaly

With the publication of the JPL’s definitive 2002 study of the Pioneer anomaly [27Jump To The Next Citation Point] the efforts to study the discovered effect have intensified. After some hesitation there was an agreement among the researchers that a thorough investigation of the anomaly is needed. The main question remained: What is the nature of the Pioneer anomaly? Is this a manifestation of new physics? Or a representation of some unmodeled conventional physics mechanism? Furthermore, it became clear that before the Pioneer anomaly can be fully accepted as a novel physical effect, the issue of the on-board heat redistribution must be fully addressed.

Ultimately, the search for the origin of the Pioneer anomaly came down to one principal question: “Is it the heat or not the heat?” Multiple community efforts are now guided by this question. In addition to the teams from JPL and The Aerospace Corporation, several independent teams and researchers have confirmed the existence of the anomalous acceleration in the Pioneer Doppler data; the new analyses also studied temporal behavior of the anomaly, establishing limits on the temporal change of anomalous acceleration. As a result of these extensive multi-year efforts, our understanding of the Pioneer anomaly improved significantly (see Section 7.1).

Meanwhile, a team at JPL along with C.B. Markwardt at the NASA Goddard Space Flight Center have been working on the recovery of a much extended radiometric Doppler data set, covering a significantly longer time period for both spacecraft than previously available data (Section 3.3). At the same time, thanks mainly to the efforts of L.R. Kellogg at the NASA Ames Research Center, a near complete record of telemetry from Pioneer 10 and 11 (Section 3.5) has been recovered, as well as significant quantities of original project documentation (Section 2.1.3). This made it possible to construct a detailed thermal model of the spacecraft, investigating the extent to which heat generated on-board and radiated away anisotropically may contribute to the anomalous acceleration (Section 7.4).

As of early 2010, some of these efforts are finished, such as the recovery of the Doppler data, while others, notably, the construction of a comprehensive thermal model, are still under way. This new effort to investigate the Pioneer anomalous acceleration, when completed, will for the first time use all available Doppler data to establish the acceleration profile of the two spacecraft, and it will also accurately account for any acceleration of on-board thermal origin.

In this section, we review these recent and on-going efforts and summarize our current knowledge of the Pioneer anomaly.

 7.1 Independent verifications
  7.1.1 Independent verification by Markwardt
  7.1.2 Analysis by Olsen using HELIOSAT
  7.1.3 Independent analysis by Toth
  7.1.4 Analysis by Levy et al. using ODYSSEY
 7.2 Unresolved questions
  7.2.1 Direction
  7.2.2 The magnitude and temporal behavior of the effect
  7.2.3 Onset of the anomaly
  7.2.4 Annual/diurnal terms
  7.2.5 Radio beam reaction force
  7.2.6 The Pioneer spin anomaly
  7.2.7 Thermal recoil forces
 7.3 An approach to finding the origin of the Pioneer anomaly
  7.3.1 Analysis of the earlier trajectory phases
  7.3.2 Study of the planetary encounters
  7.3.3 Study of the temporal evolution of the anomaly
  7.3.4 Analysis of the individual trajectories for both Pioneers
  7.3.5 Investigation of on-board systematics
 7.4 The thermal recoil force
  7.4.1 General formalism
  7.4.2 Contribution of the heat from the RTGs
  7.4.3 Effects of the electrical heat
  7.4.4 The thermal recoil force

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