Dubbed the Pioneer Anomaly, the unexplained force appears to be acting against NASA’s identical Pioneer 10 and 11 probes, holding them back as they head away from the Sun.
Whether that force stems from the probes themselves, something exotic like dark matter, or some new facet of physics or gravity, remains in doubt.
But a wealth of newly recovered data and telemetry, spanning decades of observations by both Pioneer 10 and 11, may yield the final answer to whether conventional physics or perhaps something new is at work on the two spacecraft. An answer could arise from the new data after about a year of analysis by an international team of researchers.
“I would like to see this story reach its finality,” said Slava Turyshev, an astrophysicist with NASA’s Jet Propulsion Laboratory (JPL) who has spent the last 14 years—some of it on his own time—studying the Pioneer Anomaly. "So if it’s conventional physics, that’s fine and we can all go about our daily business. But if it’s something else, there may be another page.” . . .
Just to throw in something to the tune of the "new facet of gravity" approach, the acceleration the Pioneers seem to be experiencing is, oddly enough, pretty close to the acceleration scale introduced in Modified Newtonian Dynamics, the idea that suggests changes to Newton's laws are required (instead of dark matter) to explain the dynamics of galaxies. There's a tiny bit on that in this thread.