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 ∼8.5×10−8 cm/s2, 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.Comment: Revtex, 4 pages and 1 figure. Minor changes for publicatio