Heard about this for the first time today while listening to the audiobook version of “13 things that do not make sense” by ex New Scientist editor Michael Brooks.
From wikipedia:
The Pioneer anomaly or Pioneer effect is the observed deviation from predicted trajectories and velocities of various unmanned spacecraft visiting the outer solar system, most notably Pioneer 10 and Pioneer 11.
Both Pioneer 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.
The anomaly has no universally accepted explanation. The explanation may be mundane, such as measurement error, thrust from gas leakage or uneven radiation of heat. However, it is also possible that current physical theory does not correctly explain the behaviour of the craft relative to the sun.
