7.3 An approach to finding the origin of the Pioneer anomaly

With the availability of the new Pioneer 10 and 11 radiometric Doppler data (see Section 3.3), a new study of the Pioneer anomaly became possible. This much extended set of Pioneer Doppler data is the primary source for the new ongoing investigation of the anomaly. In addition, the entire record of flight telemetry files received from Pioneer 10 and 11 is also available (see Section 3.5). Together with original project documentation (see Section 2.1.3) and newly developed software codes and trajectory analysis tools, this additional information is now used to reconstruct the engineering history of both spacecraft with the aim to establish the nature of the Pioneer anomaly. Below we review the current status of these efforts.

The primary objective of the new investigation is to determine the physical origin of the Pioneer anomaly and identify its properties. To achieve this goal, a study of the recently recovered radiometric Doppler and telemetry data has begun, focusing in particular on improving our understanding of the thermal behavior of the spacecraft and the extent to which radiated heat can be responsible for the acceleration anomaly.

The objectives of this new investigation of the Pioneer anomaly are sixfold:

To analyze the early mission data; the goal would be to determine the true direction of the anomaly and thus, its origin;
To study the physics of the planetary encounters; the goal would be to learn more about the onset of the anomaly (e.g., Pioneer 11’s Saturn flyby),
To study the temporal evolution of aP with the entire data set; the goal would be a better determination of the temporal behavior of the anomaly,
To perform a comparative analysis of individual anomalous accelerations for the two Pioneers with the data taken from similar heliocentric distances, which could highlight properties of aP, and
To investigate the on-board systematics with recently recovered MDRs; the goal here would be to investigate the effect of on-board systematics on the solution for the Pioneer anomaly obtained with the Doppler data, and, finally
To build a model of the thermal, electrical and dynamical behavior of the Pioneer vehicles and verify it with the actual data from the MDRs; the goal here would be to develop a model to be used to calibrate the Doppler anomaly with respect to the on-board sources of dynamical noise.

These objectives are not entirely independent of each other; by putting them on this list, we are identifying the main areas that are the focus of the on-going new investigation of the anomaly. Below we will discuss these objectives in more detail.

7.3.1 Analysis of the earlier trajectory phases

One objective of the new investigation is the study of the early parts of the trajectories of the Pioneers with the goal of determining the true direction of the Pioneer anomaly and possibly its origin [260391Jump To The Next Citation Point393Jump To The Next Citation Point]. The much longer data span is expected to improve the ability to determine the source of the acceleration. In particular, with data from much closer to the Earth and Sun, one should be able to better determine whether the acceleration is i) in the sunward direction, ii) in the Earth-pointing direction, iii) in the direction along the velocity vector, or iv) along the spin axis direction (see Section 7.2.1). Analysis of the earlier data is critical in helping to establish a precise 3-dimensional time history of the effect, and therefore to find out whether it is due to a systematic or new physics.

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Figure 7.5: Proposed directions (along the spin and antenna axes) from the Pioneer F spacecraft (to become Pioneer 10) toward the Earth [337Jump To The Next Citation Point].
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Figure 7.6: The earlier part of the Pioneer 10 trajectory before Jupiter encounter, the part of the trajectory when antenna articulation was largest [337].

7.3.2 Study of the planetary encounters

An approximately constant anomalous acceleration seems to exist in the data of Pioneer 10 as close in as 27 AU from the Sun [24Jump To The Next Citation Point27Jump To The Next Citation Point269]. Navigational data collected for Pioneer 11, beginning just after Jupiter flyby, show a small value for the anomaly during the Jupiter-Saturn cruise phase in the interior of the solar system. However, right at Saturn encounter, when Pioneer 11 passed into a hyperbolic escape orbit, there was an apparent fast increase in the anomalous acceleration [27Jump To The Next Citation Point255393Jump To The Next Citation Point], but this has not yet been confirmed by rigorous analysis (see Section 7.2.3).

Doppler data covering Pioneer 11’s encounter with Saturn are available. A successful study of the data surrounding the encounter [391Jump To The Next Citation Point] would lead to an improved understanding of the apparent onset of the anomalous acceleration. The encounters of both spacecraft with Jupiter may also be of interest (see [397Jump To The Next Citation Point]), although that close to the sun, much larger contributions to the acceleration noise are present.

While early data may improve our understanding of the direction of the anomaly, a difficult obstacle exists along the way towards this goal [27Jump To The Next Citation Point260]. Radiometric observables, notably Doppler, are sensitive in the line-of-sight direction, but are insensitive to small changes in the spacecraft’s orbit in a direction that is perpendicular to the line-of-sight. The lack of a range observable on Pioneer 10 and 11 also reduces the accuracy with which the orbit can be determined in three dimensions. Nevertheless, these problems can be addressed and the on-going analysis should be able to yield the true direction of the anomaly and its origin [255391393].

7.3.3 Study of the temporal evolution of the anomaly

JPL’s 2002 analysis [27Jump To The Next Citation Point] found that the anomalous acceleration is approximately constant. On the other hand, any explanation involving the on-board thermal inventory of the spacecraft must necessarily take into account this inventory’s decay with time. It was on this basis that the authors of [27Jump To The Next Citation Point] rejected the hypothesis that the acceleration is due to collimated thermal emission.

While JPL’s study of 11.5 years of Pioneer 10 data [27Jump To The Next Citation Point28] found no change in the anomalous acceleration, Markwardt [194], Olsen [274] and Toth [377Jump To The Next Citation Point] were not able to rule out this possibility. The now available extended data set, which includes over 20 years of usable Pioneer 10 data, may be sufficient to demonstrate unambiguously whether or not a jerk term is present in the signal, and if it is compatible with the temporal behavior of the on-board thermal inventory [378]. We note, however, the difficulty of the task of disentangling such a jerk term from the effects of solar radiation pressure.

7.3.4 Analysis of the individual trajectories for both Pioneers

The trajectories of Pioneer 10 and 11 were profoundly different. After its encounter with Jupiter, Pioneer 10 continued on a hyperbolic escape trajectory, leaving the solar system while remaining close to the plane of the ecliptic. Pioneer 11, in contrast, proceeded from Jupiter to Saturn along a trajectory that took it closer to the Sun, while outside the ecliptic plane. After its encounter with Saturn, Pioneer 11 also proceeded along a hyperbolic escape trajectory, but once again it was flying outside the plane of the ecliptic. In the end, the two spacecraft were flying out of the solar system in approximately opposite directions.

Nonetheless, the limited data set that was available previously precluded a meaningful comparison. The individual solutions for the two spacecraft were obtained from data segments that not only differed in length (11.5 and 3.75 years), but were also taken from different heliocentric distances (see Section 5.6).

From the recovered telemetry [397Jump To The Next Citation Point] we now also know that the actual thermal and electrical behavior of the two spacecraft was different [378379397Jump To The Next Citation Point]. These facts underline the importance of studying and comparing the behavior of both spacecraft, as this may help determine if the anomaly is of on-board origin or extravehicular in nature.

7.3.5 Investigation of on-board systematics

The availability of telemetry information makes it possible to conduct a detailed investigation of the on-board systematic forces as a source of the anomalous acceleration.

Previously, all known mechanisms of on-board systematic forces were examined [24Jump To The Next Citation Point262527Jump To The Next Citation Point164245327397Jump To The Next Citation Point] (see Table 5.2). Current efforts are designed to improve our understanding of the contribution of on-board heat – notably, heat from the RTGs reflecting off the spacecraft, and electrical heat generated within the spacecraft – to the anomalous acceleration. The available telemetry also helps refine estimates of the radio beam reaction force. (Other effects, such as the differential emissivity of the RTGs, helium expulsion from the RTGs, propulsive gas leaks, were also analyzed [379397Jump To The Next Citation Point] but were found to be insignificant.)

As pointed out in [28], any thermal explanation should clarify why either the radioactive decay (if the heat is directly from the RTGs/RHUs) or electrical power decay (if the heat is from the instrument compartment) is not seen. One reason could be that previous analyses used only a limited data set of only 11.5 years when the thermal signature was hard to disentangle from the Doppler residuals or the fact that the actual data on the performance of the thermal and electrical systems was not complete or unavailable at the time the analyses were performed.

The present situation is very different. Not only do we have a much longer Doppler data segment for both spacecraft, we also have the actual telemetry data on the thermal and electric power subsystems for both Pioneers for the entire lengths of their missions. The electrical power profile of the spacecraft can be reconstructed with good accuracy using electrical telemetry measurements (see Section 2, and also [397Jump To The Next Citation Point]). The telemetry also contains measurements from a large number of on-board temperature sensors.

This information made it possible to construct a detailed thermal model of the Pioneer spacecraft (see Figures 7.7View Image and 7.8View Image). As of early 2010, this work is near completion, and its results are being readied for publication.

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Figure 7.7: A geometric model (left) of the Pioneer spacecraft, used for finite element analysis, and a photograph (right) of Pioneer 10 prior to launch. The geometric model accurately incorporates details such as the Medium Gain Antenna (MGA), the Asteroid-Meteoroid Detector, and the star sensor shade. Note that in the geometric model, the RTGs are shown in the extended position; in the photograph, the RTGs are stowed. From [171Jump To The Next Citation Point172Jump To The Next Citation Point].
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Figure 7.8: A “work-in-progress” temperature map of the outer surface of the Pioneer 10 spacecraft body, comparing temperatures calculated via a numerical finite element method vs. temperatures measured by platform temperature (PLT) sensors and telemetered. While agreement between calculated and telemetered temperatures is expected to improve as the model is being developed, discrepancies between these values illustrate the difficulties of creating a reliable temperature map using numerical methods. (From [171Jump To The Next Citation Point172Jump To The Next Citation Point]).

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