22 Apr '12 08:04

The various exotic explanations for Pioneer's anomalous trajectory have now been ruled out in favour of a more mundane explanation:

http://arxiv.org/abs/1204.2507

the above doc is only a brief summary. If you want more detail, you would need to go to:

http://physicsworld.com/cws/article/news/2012/apr/16/exotic-explanation-for-pioneer-anomaly-ruled-out

-but you would need to sign-up to view that.

http://arxiv.org/abs/1204.2507

Support for the thermal origin of the Pioneer anomaly

…..

(Submitted on 11 Apr 2012)

We investigate the possibility that the anomalous acceleration of the Pioneer 10 and 11 spacecraft is due to the recoil force associated with an anisotropic emission of thermal radiation off the vehicles. To this end, relying on the project and spacecraft design documentation, we constructed a comprehensive finite-element thermal model of the two spacecraft. Then, we numerically solve thermal conduction and radiation equations using the actual flight telemetry as boundary conditions. We use the results of this model to evaluate the effect of the thermal recoil force on the Pioneer 10 spacecraft at various heliocentric distances. We found that the magnitude, temporal behavior, and direction of the resulting thermal acceleration are all similar to the properties of the observed anomaly. As a novel element of our investigation, we develop a parameterized model for the thermal recoil force and estimate the coefficients of this model independently from navigational Doppler data. We find no statistically significant difference between the two estimates and conclude that once the thermal recoil force is properly accounted for, no anomalous acceleration remains.

the above doc is only a brief summary. If you want more detail, you would need to go to:

http://physicsworld.com/cws/article/news/2012/apr/16/exotic-explanation-for-pioneer-anomaly-ruled-out

-but you would need to sign-up to view that.