The Pioneer Anomalous Acceleration

Meta Research as an organization is always on the lookout for anomalies, and much of our research is directed by them. Of course, most anomalies turn out to have prosaic explanations. But it is the few that do not that have the potential to point us in the direction of new discoveries. Nonetheless, it seems important to assure our members that we are not unmitigated anomaly-proponents, and we rule out far more of them than the few we find occasion to report on. One such that has received considerable attention, especially on the internet, is the so-called “Pioneer anomaly”, an excess radial acceleration of the two Pioneer spacecraft toward the Sun with unknown origin.

In investigating this anomaly, we first noted that the spacecraft unmodeled accelerations are so relatively large that they must be unique to very small bodies because the planets, moons, comets, and asteroids at any distance do not share these accelerations. Something of a similar (but not quite the same) size is seen in the Ulysses spacecraft with an orbit near Jupiter’s, but not in the Voyagers or other spacecraft. Yet the Pioneer anomalous acceleration is of a magnitude such as might be produced by an undiscovered planet inside the orbit of Neptune with a mass of 1.5 Jupiters, or more than 0.1% of the Sun’s own acceleration at the distance of the Pioneer spacecraft. The distinguishing characteristic of gravitational forces is that they induce the same acceleration in masses of all size. Therefore, whatever the anomalous acceleration may be, it is definitely not gravitational, and appears to be specific to certain spacecraft.

A redshift interpretation is ruled out because a Hubble-rate-size change in all Earth clocks is inconsistent with pulsar timing data, which shows no consistent acceleration. Interstellar winds would be highly direction-dependent and affect only small masses, but the effect would be much smaller than either Pioneer sees, and unable to affect the Ulysses spacecraft. Hubble expansion, dark matter, and a variety of other exotic mechanisms are also inconsistent with the data. We also note that the effect is not uniquely associated with orbits of high eccentricity because several high-eccentricity asteroid orbits (e.g., Icarus) do not show it.

A paper in the Los Alamos archive seems to explain the puzzle. See link. In brief, the anomalous acceleration may arise from radiation of waste heat by the on-board spacecraft RTG power generator. The Earth-pointing orientation of the distant spacecraft is what makes the force appear radial to the Sun. The waste heat explanation, or some variant of it, is surely correct because it matches the circumstances (choice of spacecraft affected, strength and specific signature of anomalous acceleration) so well. The waste heat itself is in excess of 2000 watts, and the anomaly requires an asymmetry in the dumping of that heat of just 60 watts, or about 3%. Those familiar with the spacecraft configuration can already account for about half that much asymmetry. Surely, some unnoticed factor will explain the remaining asymmetry because the alternative is that each of these particular spacecraft is affected by two different anomalies having a similar magnitude and direction. In any case, it seems safe to conclude that this anomaly is of interest to spacecraft trackers only, and not a symptom of some interesting astrophysical discovery still to be made.