Since the initial announcement of the anomalous acceleration of the Pioneer 10 and 11 spacecraft, a significant number of proposals have been made in an attempt to explain the nature of the discovered effect. The explanations targeted the effect with the properties presented in [24, 27, 390] and summarized in Section 5.6. These key properties include i) the magnitude and the apparent constancy of the anomalous acceleration, ii) its nearly Sun-pointing direction, and iii) the apparent “onset” of the anomaly. This set of “known” properties was used to analyze the mechanisms that were put forward in numerous attempts to identify the origin of the effect. Although the proposals are all very different and include conventional and new physics ideas, it is possible to place them into several broad categories. In this section we review some of these proposed mechanisms.
First, there are attempts to explain the anomaly using unmodeled conventional forces with an origin external to the spacecraft (Section 6.1), which may be both gravitational or nongravitational in nature. Some authors considered the possibility that the anomalous effect may be due to a new physics mechanism indicating, for instance, modification of gravity (Section 6.2), or may have a cosmological origin (Section 6.3). On the other hand, the fact that the Pioneer anomaly was observed in the radiometric Doppler signal opens up the possibility that the anomaly is not a dynamical effect on the trajectories of the probes but instead is due to an unmodeled effect on their radio signal (Section 6.4). We also consider proposals that attempt to explain the anomaly using unmodeled forces of on-board origin (Section 6.5). Lastly, we review miscellaneous mechanisms (Section 6.6) and some common misconceptions before moving on to a discussion of independent observational confirmations (Section 6.7) and proposals for dedicated space experiments (Section 6.8).
This work is licensed under a Creative Commons License.