Simple explanations for the Pioneer Anomaly, dating back to the late 1990s, looked at non-gravitational forces produced by the spacecraft itself due to thermal and electrical sources. Heat from Pioneer's electronics is 100 Watts. The heat from the radioactive plutonium-238 power source on Pioneer outputs 2.5 kilowatts. The nuclear "battery" is on a boom extending from one side of the 550-pound spacecraft. This would cause "thermal recoil" as one side of the vehicle was slightly warmed (thermal model above).
Slava Turyshev of NASA's Jet Propulsion Laboratory proposed the recoil theory several years ago. Since then he has extract more archival data from Pioneer's tracking. The smoking gun, as described in a recently published paper that the data show a drop in Pioneer's anomalous motion. This is exactly what would be predicted if thermal heating is the culprit. The 10 pounds of plutonium aboard Pioneer cools as it decays exponentially.
Both Pioneer 11 and 12 would show exactly the same anomaly because they are identically built. But what about testing other spacecraft?
The New Horizons probe blazing its way to Pluto should also have peculiarities due to heat from its nuclear power source, though its tracking is not a precise as for the Pioneers. The two Voyager spacecraft are less sensitive to the effect seen on Pioneer, because their thrusters align it along three axes, whereas the Pioneer spacecraft rely on spinning to stay stable. Other solar system spacecraft are in the wrong orbit, have larger nuclear power sources, and do frequent maneuvers.
"For the foreseeable future Pioneer 10 and 11 remain the largest scale precision gravitational experiment ever conducted," wrote Victor Toth (Perimeter Institute for Theoretical Physics, Waterloo, Ontario Canada) in 2011. "Far more likely this (Pioneer anomaly) was just a wild goose chase."
Lessons learned are that there are limits to our tracking and navigational accuracy, it is critical to archive long-term data on spacecraft, and estimates of small forces acting on a spacecraft really need to be precisely done.
HOWSTUFFWORKS: Pioneer 10, Flight to Jupiter (1972-1997)
The Pioneer Anomaly is a case study where the easiest explanation for a phenomenon is usually the correct one, and there is no need for overturning the laws of physics.
This is a time honored maxim call Occam's Razor: when you have two competing theories that make exactly the same predictions, the simpler one is the most likely solution.
This no doubt has parallels to the faster-than-light neutrino detection reported earlier this year. It got a lot of attention but appears to have a mundane solution too.
Nobody said that all strange mysteries we uncover in the universe need to have, well, strange answers.
Finally, to borrow from Henry Wadsworth Longfellow:
I shot a space probe into the sky,
Image credit: NASA, S. Turyshev