Our paper on the analysis of extended Pioneer 10 and 11 Doppler data was just accepted by Physical Review Letters.
In it, we report that a data set roughly twice in size the data set that was analyzed previously continues to support the notion that a small anomalous acceleration is affecting both spacecraft. However, there is no reason to believe that the acceleration is in the direction of the Sun or that it is constant; on the contrary, the data seem to favor (albeit weakly) an Earth-directed, temporally decaying acceleration model.
Heat, emitted anisotropically, remains the prime suspect. The observed decrease in the acceleration appears more rapid than the rate of decay of the radioactive fuel on board. This is explained once we consider that much of the thermal acceleration would be due to electrically produced heat, and the amount of electricity available on board decreases much more rapidly. (The reason is that as the plutonium fuel cools, the thermocouples used to generate electricity become less efficient; the thermocouples also age.)
We also looked at some early data, taken when Pioneer 11 was cruising between Jupiter and Saturn. The possibility that the anomalous acceleration only began after Pioneer 10 and 11 passed the orbit of Saturn was much discussed in the literature. While we cannot exclude such an onset, its presence cannot be confirmed either (the early data is just too short in duration for a definitive conclusion). In any case, the shape of the onset curve very strongly suggests that it is, in fact, a modeling artifact: it is precisely what one would see if the Pioneer spacecraft’s solar pressure model was miscalibrated, which is a very likely possibility.
In the past few years, we also constructed a detailed thermal model of the Pioneer spacecraft, using recovered documentation and telemetry. We are busy preparing another report in which the results of this effort will be discussed.
