Indication, from Pioneer 10/11, Galileo, and Ulysses Data, of an   Apparent Anomalous, Weak, Long-Range Acceleration
  Radio metric data from the Pioneer 10/11, Galileo, and Ulysses spacecraft indicate an apparent anomalous, constant, acceleration acting on the spacecraft with a magnitude $\sim 8.5\times 10^{-8}$ cm/s$^2$, directed towards the Sun. Two independent codes and physical strategies have been used to analyze the data. A number of potential causes have been ruled out. We discuss future kinematic tests and possible origins of the signal. 
John D. Anderson, Philip A. Laing, Eunice L. Lau, Anthony S. Liu,   Michael Martin Nieto, and Slava G. Turyshev