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).

6.1 Unmodeled forces external to the spacecraft

6.1.1 Gravitational forces due to unknown mass distributions and the Kuiper belt

6.1.2 Drag forces due to interplanetary dust

6.2 Possibility for new physics? Modified gravity theories

6.2.1 Dark matter

6.2.2 Modified Newtonian Dynamics (MOND)

6.2.3 Large distance modifications of Newton’s potential

6.2.4 Scalar-tensor extensions of general relativity

6.2.5 Scalar-tensor-vector modified gravity theory (MOG)

6.3 Cosmologically-motivated mechanisms

6.3.1 Cosmological constant as the origin of the Pioneer anomaly

6.3.2 The effect of cosmological expansion on local systems

6.3.3 The cosmological effects on planetary orbits

6.3.4 Gravitationally bound systems in an expanding universe

6.3.5 Dark-energy-inspired gravity models

6.4 Effects on the radio signal

6.4.1 Helicity-rotation coupling

6.4.2 Clock acceleration

6.4.3 Cosmological effects on radio signals

6.5 Unmodeled forces of on-board origin

6.5.1 Thermal recoil forces

6.5.2 Propulsive gas maneuvers

6.6 Miscellaneous mechanisms

6.6.1 Other proposed mechanisms

6.6.2 Common misconceptions and numerical coincidences

6.7 Search for independent confirmation

6.7.1 Solar-system planetary data analysis

6.7.2 Attempts to confirm the anomaly using other spacecraft

6.7.3 Possible future spacecraft searches

6.8 A mission to explore the Pioneer anomaly

6.8.1 A Pioneer instrument package

6.8.2 A dedicated mission concept

6.1.1 Gravitational forces due to unknown mass distributions and the Kuiper belt

6.1.2 Drag forces due to interplanetary dust

6.2 Possibility for new physics? Modified gravity theories

6.2.1 Dark matter

6.2.2 Modified Newtonian Dynamics (MOND)

6.2.3 Large distance modifications of Newton’s potential

6.2.4 Scalar-tensor extensions of general relativity

6.2.5 Scalar-tensor-vector modified gravity theory (MOG)

6.3 Cosmologically-motivated mechanisms

6.3.1 Cosmological constant as the origin of the Pioneer anomaly

6.3.2 The effect of cosmological expansion on local systems

6.3.3 The cosmological effects on planetary orbits

6.3.4 Gravitationally bound systems in an expanding universe

6.3.5 Dark-energy-inspired gravity models

6.4 Effects on the radio signal

6.4.1 Helicity-rotation coupling

6.4.2 Clock acceleration

6.4.3 Cosmological effects on radio signals

6.5 Unmodeled forces of on-board origin

6.5.1 Thermal recoil forces

6.5.2 Propulsive gas maneuvers

6.6 Miscellaneous mechanisms

6.6.1 Other proposed mechanisms

6.6.2 Common misconceptions and numerical coincidences

6.7 Search for independent confirmation

6.7.1 Solar-system planetary data analysis

6.7.2 Attempts to confirm the anomaly using other spacecraft

6.7.3 Possible future spacecraft searches

6.8 A mission to explore the Pioneer anomaly

6.8.1 A Pioneer instrument package

6.8.2 A dedicated mission concept

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