Wasn't it who wrote:
http://www.youtube.com/watch?v=AoEfWNkdScY mentions that these
satellites (?) probes rather, wondered off course and no-one knows
why.

Anyone have a theory?

Last I heard as that the tiny deviation from the predicted course could
be accounted for by the difference in radiation pressure caused by the
far side being darker than the near side and therefore emitting more IR.

When the anomaly was first noticed, back in 1998, there were some whacky
suggestions put forth that gravity might not exactly follow an inverse-
square law, but that's been pretty well squashed by noticing that no
such effects have been observed on other probes such as Galileo and the
Voyagers. Whatever is causing the effect is specific to the design of
the Pioneer probes.

--
Mike Williams
Gentleman of Leisure