Mystery Force Affecting Probes

imipak writes: "The BBC reports that after exchaustive investigations, NASA scientists have run out of possible explanations for the mysterious tiny course deflections experienced by the Pioneer and Voyager spacecraft as they head out of the solar system towards the heliopause. Could it be that there's something wrong with our theory of gravity? (Well, yes, we already know that...) or could it be Oort Cloud objects? The tenth planet? Informed comment, please!"