Pioneer Anomaly: A Long-Distance Bill

Scientific American closes in on the “Pioneer Anomaly” – the strange fact that the Pioneer deep space probes aren’t traveling at exactly the right speed. Researchers now think it might be a problem with plutonium:

A group of some 50 researchers, including Turyshev, is now trying to match the data to a detailed computer model of the craft’s inner workings. The model is designed to mimic the flow of heat and electricity produced by the craft’s generators, which harnessed the heat from radioactive plutonium and turned a fraction of it into electricity to power the craft. The remaining heat [see note below] was lost to space or spread to other parts of the craft such as the antenna, which influenced each probe’s overall momentum.

So far the model accounts for about 30 percent of the observed anomaly for Pioneer 10 at a single distance of 25 astronomical units (2.3 billion miles, or 3.7 billion kilometers) from the sun, Turyshev reported. The group still has to extend the model to other distances and to Pioneer 11. The full verdict may not be in for some time.

Why does this matter? Well, the Pioneer Anomaly might also have something to do with the way gravity shapes the universe, and the way interstellar craft can travel over very long distances.