Credit: NASA
But not all of the homework has been done. Another mundane explanation is that the anomaly is due to waste heat from parts of the probe facing away from the Sun.
It's another light-pressure effect, although in this case it's the probe, not the Sun, that's emitting the light. In this case, the light is infrared radiation, and while that is less energetic than visible light, it too would cause the spacecraft to recoil slightly in the opposite direction.
Some of it might also bounce off the back side of the probe's antenna, causing it to act like a tiny light sail.
One source of heat is from the probes' RTGs (radioisotope thermoelectric generators), which are electrical generators powered by heat from radioactive decay. These were designed to radiate heat equally in all directions, but if they didn't quite do so, the disparity might produce enough acceleration.
But again, there's a problem. The power from RTGs falls off in tandem with the radioisotopes' half-lives. And that, Anderson says, isn't what the scientists have been seeing.
Still, Turyshev says, preliminary assessments by Gary Kinsella at JPL found that there is enough heat from the RTGs to account for at least part of the anomaly. "What part, we don't know yet," Turyshev says. "Basically, there is no number yet."
More importantly, during Anderson's initial studies, there were only 11.5 years of data from Pioneer 10, and 3.25 years of data from Pioneer 11.
Thanks to work by Viktor Toth, a Hungarian-Canadian software engineer (who, like a number of people studying the anomaly, works on the problem in his spare time), old records have been located and digitised, allowing the probes' flight paths to be examined in much greater detail.
"Now we have roughly 20 years of Pioneer 11 and maybe 18 years of Pioneer 10," Turyshev says. And in addition to the Doppler data, his team recovered data from 114 on-board sensors. That means more than simply modelling heat loss from the RTGs; it permits modelling of infrared emissions from other equipment in exquisite, day-to-day detail.
"That information can be used to build a high-fidelity thermal model of the spacecraft," Turyshev says. With that information in hand, the scientists can then create a computer model that will 'fly' the two spacecraft "year by year and manoeuvre by manoeuvre," he says.
It's likely that there will still be an anomaly, but at least this will further refine our estimates and possibly reveal part of the cause, Turyshev says. "That's a guesstimate. We should have results in about 10 months."
An obvious question is why subsequent probes to the outer Solar System haven't shown the same effect. Sadly, these craft weren't designed to look for it.
Some, like Galileo, simply didn't go far enough out. Others, like the Voyager or Cassini missions, used thrusters for attitude control, rather than being spin-stabilised (or spun about an axis to maintain orbital control, similar to a gyroscope), as the Pioneers were.
Uncertainties in the power of thruster bursts are enough to overwhelm any Pioneer-style anomalies in the trajectories of the other probes.
