Thanks to funding from the Planetary Society, of which I am a proud member, the “Pioneer Anomaly” has been definitively resolved. Physics has prevailed over speculation.
Over the past 20 years, some people, including a few scientists, have looked at oddities in the trajectories of the Pioneer 10 and 11 spacecraft and proposed that our understanding of gravity was flawed. Now we have a much more mundane explanation that requires no exotic new theories.
Due to the structure of the spacecraft, they radiate more heat in some directions than others. That produces a very small force that, when added to the gravity of known solar system bodies, leads to exactly the trajectories that have been observed.
It took a lot of detailed sleuthing to get the necessary data, but the result is satisfying, even though it disappoints some who were hoping for a breakthrough that might lead us to resolving some perplexing issues in physics. [See the prologue and concluding sections of Physics: Decade by Decade by Fred Bortz (Twentieth-Century Science set, Facts on File, 2007).]