Pioneer has even made a cameo appearance in a Star Trek movie when the evil Klingons find it in interstellar space and shoot it for target practice.
Now over 7 billion miles from Earth (10 light-hours) Pioneer 11 and 12 serve as "test particles" for measuring the effects of gravity on manmade objects over very large distances. Such a test has never before been possible.
In the 1980s several research teams independently measured what was interpreted as an infinitesimal deceleration of both Pioneers, which are streaking away from us in nearly opposite directions. The amount was inconsequential by engineering standards, but a huge discrepancy in predictions made by the laws of gravity.
The direction of the anomalous force had also come under question: is it really in the sun's direction, or Earth's, or along the spacecraft's spin axis or velocity direction?
Scientists began toying with the exotic theories for explaining the anomaly. Perhaps the laws of gravity needed to be modified. Was dark matter in our local neighborhood tugging on the Pioneers? Or did it have an even deeper implication for cosmology? One idea was that a localized blob of dark matter could be trapped in the sun's gravitational field. Any effects from dark energy would be way to small for explaining the Pioneer motion.