Re-examination of Pioneer 10 tracking data including spacecraft thermal modeling

The so-called Pioneer Anomaly is an anomalous shift in the radio-metric track-ing signals received by Pioneer 10 and 11 spacecrafts, which can be ascribed to an approximately constant sunward acceleration acting on the probes. This ano-malous acceleration has a similar magnitude for the two spacecraft, firstly re-ported by Anderson and al. in 1998, to be (8.74±1.33)∙10-10 m/s2 (Reference 2). Since then, the existence of the anomaly has been confirmed independently by several groups and a large effort was devoted to find its origin providing, how-ever, no definitive answers. These attempts can be referred to two quite different approaches: the first one, seeking for “conventional” (i.e. engineering) explana-tions, encompasses unaccounted on-board and environmental systematic effects; the other one, recognizing the anomaly as a deviation from Einstein's gravity theory, seeks for new fundamental physics models. The present study, carried out at the Radio Science lab of the University of Bologna in Forlì consists of two main parts: a detailed thermal modeling of the spacecraft throughout its trajectory, and orbit determination analysis. Indeed, anisotropic thermal radiation is one of the most promising candidates to account for, at least partially, the anomaly. Based on existing documentation4 and recov-ered telemetry data9, we built a finite element model of the spacecraft, whose complexity has been constrained to a degree allowing for a sensitivity analysis. The trajectory analysis has then been performed using JPL’s ODP (Orbit Deter-mination Program) available to the University of Bologna, thanks to a NASA/ASI bilateral agreement. Results show that orbital solutions may be achieved that do not require any anomalous acceleration other than the one of thermal origin.