Observations of asteroids around 20 AU may provide insights if the anomaly's cause is gravitational. Meetings and conferences about the anomaly A meeting was held at the University of Bremen in 2004 to discuss the Pioneer anomaly. The Pioneer Explorer Collaboration was formed to study the Pioneer Anomaly and has hosted three meetings (2005, 2007, and 2008) at International Space Science Institute in Bern, Switzerland, to discuss the anomaly, and discuss possible means for resolving the source. Notes See also Flyby anomaly References Further reading The original paper describing the anomaly A lengthy survey of several years of debate by the authors of the original 1998 paper documenting the anomaly. The authors conclude, "Until more is known, we must admit that the most likely cause of this effect is an unknown systematic. (We ourselves are divided as to whether 'gas leaks' or 'heat' is this 'most likely cause.')" The ISSI meeting above has an excellent reference list divided into sections such as primary references, attempts at explanation, proposals for new physics, possible new missions, popular press, and so on. A sampling of these are shown here: Theory establishes a gravitational connection between the unexplained periastron advance observed in two binary star systems and the Pioneer anomaly. Further elaboration on a dedicated mission plan (restricted access) Popular press – STVG (Scalar-tensor-vector gravity) theory claims to predict Pioneer anomaly External links Shows number of publications about the Pioneer anomaly on arXiv.org, by year. Pioneer program Astrodynamics