A quick follow-up to one of the talks I gave at Dragon*Con last year. The Pioneer Anomaly refers to how the Pioneer 10 and 11 space probes were pulled more strongly towards the sun than they should have been, making them slow down more than expected. Scientists have been trying to explain what’s going on for about fifteen years. Wikipedia has a pretty good rundown on possible explanations. But to date we still don’t know what’s really causing the Pioneer acceleration.
One theory was that our knowledge of gravity is wrong. There might be higher-order effects that only happen at very long distances, so that we don’t normally see the effect. However, any gravity weirdness like that would affect not only Pioneer 10 and 11 but also the outermost planets in the solar system. A recent paper looked at the orbits of some of Neptune’s moons. They didn’t find any evidence of the Pioneer Anomaly, which means some of the tweaks to gravity like MOND can’t explain the anomaly.
Caution: you may want to skip the comments at the bottom of the linked article, as there’s some crankpotulous discussions going on.