The Pioneer anomaly or Pioneer effect is the observed deviation from expectations of the trajectories of various unmanned spacecraft visiting the outer solar system, notably Pioneer 10 and Pioneer 11. Both spacecraft are escaping from the solar system, and are slowing down under the influence of the Sun's gravity. Upon very close examination, however, they are slowing down slightly more than expected. The effect can be modeled as a slight additional acceleration towards the Sun.
At present, there is no universally accepted explanation for this phenomenon; while it is possible that the explanation will be mundane—such as thrust from gas leakage—the possibility of unknown physics is also being considered.
