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.