It’s looking more and more likely that the explanation for the so-called “Pioneer anomaly” — a mysterious deceleration of NASA’s Pioneer 10 and Pioneer 11 spacecraft in their wanderings beyond our solar system that has been puzzling researchers for the last 30 years — is not due to new physics, but to far more mundane factors.

The latest analysis by NASA researchers lends further credence to the theory that the anomaly is due to heat emitting from the spacecraft’s radioactive batteries.

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NASA launched Pioneer 10 in 1972 and Pioneer 11 the following year, part of a mission to explore the asteroid belt, Jupiter, Saturn, and their respective moons. Those “flyby” missions were completed within a few years, but the plucky little Pioneers kept going, eventually traveling beyond our solar system. And all along scientists at the Jet Propulsion Lab (JPL) have been tracking their progress through Doppler radio transmissions.

In 1980, an astronomer named John Anderson created an impossibly complicated algorithm so that he and other JPL scientists could use the radio transmission data to study gravitational effects in the outer solar system.

But it didn’t seem to work. Or rather, he noticed a small discrepancy between the Doppler shifts predicted by his algorithm, and the actual shifts being measured in the radio signals coming from the Pioneer spacecraft. The discrepancy is 10 billion times smaller than the acceleration due to gravity, but it was unmistakably there in Anderson’s calculations. (The canonical number, for those who care, is 8.74 x 10-10 m/s2.)

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A number of possible explanations have been proposed over the ensuing decades, including the possibility that gravity behaves differently at such large distances from earth — thereby requiring a modification of gravitational theory.

But over the last couple of years, evidence has been pointing more strongly to heat as the most likely culprit. Specifically, heat from the plutonium inside the spacecraft’s generators, some of which got converted into electricity while the rest of it radiated into space. If it did so unevenly, radiating more heat in one direction than in another — only a 5 percent difference is required — that might be sufficient to give rise to the Pioneer anomaly.

Viktor Toth (a software developer based in Canada) and JPL scientist Slava Turyshev wrote a comprehensive review paper in April 2010 making this claim. Proponents of alternative theories argue that if heat were the culprit, the effect should fade over time as the plutonium decayed. The original thermal calculations by JPL scientists showed this wasn’t the case

However, last year, Italian researcher Lorenzo Iorio published a paper on the arXiv arguing that the Pioneer anomaly is not likely to be due to modified gravity, based on his analysis of the orbits of three moons of Neptune: Triton, Nereid and Proteus. None showed evidence of similar perturbations.

In April 2011, Portuguese scientist Federico Francisco and colleagues at the Instituto de Plasmas e Fusao Nuclear in Lisbon, submitted the results from their own computer modeling analysis, claiming the original thermal calculations that ruled out heat as a possible explanation for the anomaly were wrong.

Specifically, Francisco et al. reworked the calculations taking into account not just how heat is emitted, but also how it gets reflected off various parts of the spacecraft.

And now we have JPL’s Slava Truysev and colleagues reporting on their new analysis with double the tracking data, gleaned from an exhaustive search of JPL’s records. This new, improved analysis confirms that the deceleration anomaly is most certainly occurring.

What’s more interesting is that, contrary to the original analysis conducted all those years ago, the deceleration does seem to be decelerating at an exponential rate — just as one might expect from the radioactive decay of plutonium-238, which powers the two spacecraft. Turyshev concludes, “The most likely cause of the Pioneer anomaly is the anisotropic emission of on-board heat.”

NASA is not yet going so far as to declare the mystery 100% solved — the agency is performing its own computer simulation to compare to the new dataset — but it’s looking like a 30-year mystery is drawing very near to a close.