Everybody loves a mystery, and the one surrounding Pioneer has everything going for it, an unusual effect observed via two of the most distant spacecraft ever launched. Both Pioneer 10 and 11 are slowing a bit more than expected as they move through the outer reaches of the Solar System. Explanations range from a variety of on-board causes to suggestions that our understanding of gravity itself needs an upgrade.

NASA’s Slava Turyshev, as noted in this New Scientist story, is compiling data from the spacecraft that had previously been inaccessible due to data formats and media incompatible with modern equipment. Turyshev’s work may take a year to complete, but it holds the promise of nailing what many think to be the probable cause of the anomaly: heat from the RTGs (radioisotope thermoelectric generators) that provide power for the probes. Asymmetrical radiation of that heat just might do the trick.

Meanwhile, New Scientist also gets into far more exotic possibilities, noting that ways of ‘tweaking’ gravity to account for the Pioneer anomaly don’t gibe with the observed orbits of the outer planets:

If you allow violations of [Einstein’s] equivalence principle, modifying the laws of physics can explain the Pioneer anomaly without messing up the orbits of the outer planets, says Robert Sanders of the University of Groningen in the Netherlands.

A theory called modified inertia, proposed by Mordehai Milgrom of the Weizmann Institute of Science in Rehovot, Israel, does just this. It says the way objects accelerate under gravity depends on their past trajectories – a breach of the equivalence principle. In this scenario, the Pioneer spacecraft, whose trajectories are taking them out of the solar system, experience an anomaly, while the outer planets, whose orbits keep them bound to the Sun, do not.

Fascinating stuff, but is there any need to invoke it? The odds on a more mundane explanation are enhanced by Norwegian physicist Kjell Tangen, who leaves the equivalence principle alone and finds no explanation for the Pioneer anomaly in disruptive revisions to the law of gravity. With Turyshev hot on the case, smart money will probably ride on the RTGs for now, but we have no conclusive evidence at this point, so for at least a year the odd journeys of these spacecraft will still inspire controversy.