Pioneer 10 Don Davis
Finally, the case is solved, and the villain is dead.
As the two spacecraft retreated into the distance, the data they beamed back showed that they were slowing down a little more than they should have been. Long vaunted as evidence that something was amiss in physics — perhaps that Einstein's theory of gravity was wrong — the anomaly spawned entire academic conferences and thousands of papers.
But, as explained in our coverage of an earlier stage of Turyshev et al.'s work, some scientists believed that the anomaly had a much more mundane explanation. Namely, the scientists suspected that heat was being emitted by the spacecraft's generators anisotropically — more in one direction than the other. If this were the case, the heat would exert an unbalanced recoil force on the spacecraft, causing them to change speed. Indeed, in April, a group of researchers in Portugal came up with just such a model for how the Pioneers' heaters could have created a recoil force.
But many have argued that the data itself ruled out this explanation for the Pioneer Anomaly. As the plutonium-238 that served as the Pioneers' onboard heat source radioactively decayed, it would have emitted less heat over time. Thus, if heat were the source of the Pioneer Anomaly, the anomaly should have lessened with time as well. But the data seemed to suggest that the Pioneer Anomaly was constant — an undying force — and thus much more fundamental.
But for their new analysis [PDF], Turyshev et. al. compiled a lot more data than had ever been analyzed before, spanning a much longer period of the Pioneers' flight times. They studied 23 years of data from Pioneer 10 instead of just 11, and 11 years of data from Pioneer 11 instead of 3. As explained in their new paper, the more complete data sets reveal that the spacecraft's anomalous acceleration did indeed seem to decrease with time. In short, the undying force had been dying after all, just like the decaying plutonium. In that case, it was most likely just a consequence of wonky heaters — mystery solved.