That the Pioneer spacecraft have experienced some kind of anomalous acceleration is now well established. Just what the cause of that acceleration is remains an open question. But we do know that the anomaly appears as “…a constant and uniform acceleration directed towards the Sun…”, as described in a paper called “What do the orbital motions of the outer planets of the Solar System tell us about the Pioneer anomaly?” The effect shows up in data from both spacecraft from the moment they passed the 20 AU mark on their journeys.

The authors, Lorenzio Iorio and Giuseppe Giudice (Dipartimento di Progettazione e Gestione Industriale, Naples) note that recent work has not resolved the question of whether there is some internal factor aboard the spacecraft that is causing the anomaly, or whether its origin is external. In any case, the situation is intriguing enough that dedicated space missions to explore it have been proposed.

This paper investigates whether there is an “…external, unknown constant and uniform force field…” present in the regions between 20 and 40 AU, and goes on to assess what its impact would be on the orbital motion of the outer planets. From the paper:

…it turns out that the predicted anomalous effects of this kind would be suffciently large to be well detected with the present-day level of accuracy in planetary orbit determination, even by considering errors considerably larger than those released. Their absence tell us that in the outer region of the Solar System within 20-40 AU there is no[t] any unknown force field, with the characteristics of that experienced by the Pioneer spacecraft, which affects the motion of the major planets.

If the orbital signature of the Pioneer anomaly on the outer planets is indeed within range of detection, in other words, it is significant that we do not see it. The authors’ conclusion: “This result restricts the possible causes of the Pioneer anomaly to some unknown forces which violate the equivalence principle in a very strange way or to some non-gravitational mechanisms peculiar to the spacecraft.”

Centauri Dreams‘ take: These findings would seem to put the focus back on the two spacecraft themselves, although attempts to find a solution in that direction have also been fruitless (fuel leakage triggered by thruster activity seems to be ruled out).

We may have to wait for the kind of additional data that could be provided through another space mission, perhaps one dedicated to the anomaly, as suggested by several recent researchers. But the best strategy, and centainly the less expensive, may be the one advocated by Dario Izzo and Andreas Rathke, who suggest two non-dedicated concepts that could piggyback aboard other missions (see “Options for a non-dedicated test of the Pioneer anomaly,” available here).