Launched in 1972 and 1973 respectively, the Pioneer 10 and 11 spacecraft were destined to be the first to explore the outer reaches of our solar system. We had last contact with Pioneer 10 in 2003 and it is now an astonishing 8 billion miles away. The Pioneer spacecraft have a very simple design and the 10/11 spacecraft are practically identical. It is their simplicity as compared to other craft which allowed NASA to take precise measurements of their acceleration. The Pioneer spacecraft are spin stabilized, meaning that a minimum number of thrusts are needed to maintain its position allowing for acceleration measurements with a precision of .
During the course of these measurements, the presence of a small, unexplained, constant acceleration of each spacecraft toward the Sun was noticed, with a value of . This has become known as the Pioneer anomaly.
The anomaly is measured by modeling all known effects on the craft, then taking the acceleration measured and subtracting the understood effects off. That means if we failed to account for anything in our model this could be a possible explanation for the anomaly. The explanation could be something as mundane as on board systematics of the craft or as exciting as requiring a modification of our theory of gravitational interactions to explain. Many errors on the initial model were proposed. A review paper divided into these into three categories and they are here summarized, 1. sources external to the spacecraft, 2. on board systematics and 3. computational systematics. None of these effects successfully explains the anomaly.
The Pioneer anomaly is so, well, anomalous, in part because of the fact it seems to onset after 15 AU, after which it is approximately constant. Any explanation must also explain this sudden onset which is no mean feat. Moreover, the acceleration is observed for both spacecraft which are traveling in very different directions in the universe. Any explanation, particularly systematics external to the spacecraft, must address why these systematics seem to be the same for both spacecraft in different regions of space. Finally, many tempting explanations, such as radiation pressure from the sun actually result in anomalous acceleration away from not towards the sun. Thus far, after over 7 independent studies which confirm the anomaly there have been countless others which attempt to explain it, none successfully.
My feelings, echoed by my colleagues in our journal club is that, as exciting as this pointing to a modification of our theory of gravity would be, the most likely explanation is on board systematics of the spacecraft themselves. Since the spacecraft are identical, it is conceivable that some on board effect onset at 15 AU affecting the acceleration. Unless an independently designed spacecraft verifies the anomaly this certainly seems the most plausible explanation.
1. Slava G. Turyshev, Viktor T. Toth, Larry R. Kellogg, Eunice. L. Lau, & Kyong J. Lee (2005). The Study of the Pioneer Anomaly: New Data and Objectives for New Investigation Int.J.Mod.Phys.D15:1-56,2006 arXiv: gr-qc/0512121v2
2. John D. Anderson, Philip A. Laing, Eunice L. Lau, Anthony S. Liu, Michael Martin Nieto, & Slava G. Turyshev (2001). Study of the anomalous acceleration of Pioneer 10 and 11 Phys.Rev.D65:082004,2002 arXiv: gr-qc/0104064v5
3. Slava G. Turyshev, & Viktor T. Toth (2010). The Pioneer Anomaly Living Rev. Relativity 13, 4 (2010) arXiv: 1001.3686v2