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 $\ensuremath{\sim}8.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}\mathrm{cm}/{\mathrm{s}}^{2}$, 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.