6.5 Unmodeled forces of on-board origin

The trajectory of the Pioneer 10 and 11 spacecraft may be affected by an unmodeled force of on-board origin. The Pioneer spacecraft can accelerate as a result of ejecting mass or radiation. Here we consider these mechanisms as the possible sources for the anomaly.

6.5.1 Thermal recoil forces

Immediately after publication of the initial report on the Pioneer anomaly [24Jump To The Next Citation Point], independent researchers raised questions concerning the estimated magnitude of the thermal recoil force due to internally generated heat. These ideas are summarized below.

Katz [26164] pointed out that although the electrical power generated on board Pioneer 10 and 11 was less than 100 W during the period that was investigated, the waste heat produced by each spacecraft’s four RTGs amounted to ∼ 2.5 kW. Therefore, even a small anisotropy in the spacecraft’s thermal radiation pattern may be sufficient to produce the observed acceleration. Further, the amount of heat generated on-board decreased slowly, consistent with the quoted error bounds of the acceleration. In their reply, Anderson et al. [26] reasoned that due to the geometrical features of the Pioneer spacecraft such a mechanism would produce an acceleration much smaller than that observed in the Pioneer Doppler data.

Murphy [25245] brought attention to the fact that the louver system on the back of the Pioneer spacecraft is not a Lambertian emitter, and the electrical heat produced on-board may not have been declining steadily with time, as the spacecraft body contained mostly components that had to be powered at all times. In their reply, Anderson et al. [25] argued that by the time Pioneer 10 reached the distances of ∼ 40 AU (i.e., the beginning of the data interval used in [27Jump To The Next Citation Point]), all louvers on the spacecraft were closed, thus forcing the remaining and decreasing heat to escape nonpreferentially.

Scheffer [325327] asserted that the 2002 study may have seriously underestimated the role of the thermal recoil force, and that up to 70 W of collimated thermal radiation, corresponding to an acceleration of 9.3 × 10–10 m/s2, may have been present on board the two Pioneer spacecraft. Using the data on the electrical status of the Pioneer 10 spacecraft available prior to 2002, Anderson et al. [25] argued that such a mechanism would produce a significant decrease in the magnitude of the anomalous acceleration as a response to the decreasing supply of the radio-isotopic fuel. In fact, for the Pioneer 10 data interval used in the 2002 study, the amount of electrical energy decreased by nearly 30%, implying a similar decrease in the magnitude of the anomalous acceleration. Such a decrease was not observed in the data.

Given the later recovery of i) the telemetry records of Pioneers 10 and 11 for the entire duration of their respective missions and ii) documentation detailing the design of the Pioneer F/G spacecraft (both completed in 2005 [397Jump To The Next Citation Point]), further investigation into the contribution of the thermal recoil force as the source of the Pioneer anomaly is well founded. Indeed, the recoil force due to on-board generated thermal radiation is the subject of on-going study, discussed in Section 7.4.

6.5.2 Propulsive gas maneuvers

Among the possible mechanisms for the anomaly Anderson et al. [27Jump To The Next Citation Point] emphasized that propulsive gas maneuvers could contribute to the anomalous acceleration (see discussion in Sections 4.4.1 and 5.4.1). (The topic of fuel leaks was addressed in Section 4.4.2.) The errors associated with mismodeling of the maneuvers could be large enough to seriously affect the acceleration estimates. The number of maneuvers, their magnitude and the details of the trajectories of both Pioneers make it unlikely that similar acceleration estimates for both spacecraft would result, thus undermining this idea. Recently this mechanism was again put forward by Tortora et al. [375]. The recovered telemetry record, however, shows no indication of a loss of fuel or propulsion system activity that would support this notion [397Jump To The Next Citation Point].

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