An underestimated onboard generated recoil force contributing to the Pioneer anomaly

The Pioneer anomaly, an unexpected acceleration of the Pioneer 10 and 11 spacecraft of ∼8.5 × 10−10 ms−2 directed towards the inner Solar System, has been of great interest for the physics community during the past decade: considered explanations range from new physical concepts to conventional mechanism. It is shown that non-isotropic outgassing of the complete spacecraft structure is comparable in magnitude and direction to the effect and should be considered as a significant contribution to the anomalous acceleration. Although gas leaks from e.g. the propulsion system and propulsive mass loss mechanism have been discarded as possible explanations for the anomaly, the arguments used against such mechanisms do not apply to global outgassing from the spacecraft.