The case isn’t closed yet, but it looks like the Pioneer anomaly may finally have been resolved. A recap: the Pioneer 10 and 11 spacecraft, launched in the 1970s to visit Jupiter and Saturn, have continued in a direction out of the Solar System. Scientists are still tracking their motion, and discovered to their surprise that both spacecraft are slowing down at a larger rate than expected—an anomalous deceleration.

When something like this is discovered, several options are available to scientists (more or less in order of possibility):

1. It could be that the effect isn’t really there, but is due to a calculation error. With accelerations this tiny (roughly 9 ×10^-10 m/s², which you can compare to Earth’s gravitational acceleration of 9.8 m/s²) and only two sources of data, it’s always a possibility. However, the data was precise enough to rule this option out—the effect is real.
2. The effect is real, and it’s caused by something we can understand with patience and care. In this case, the most likely culprit is heat emitted by the spacecraft from the plutonium power supply. The heat is going to preferentially be directed away from the Sun because of the orientation of the spacecraft, acting like a brake (actually a retro-rocket, which is a cooler phrase anyway). Preliminary calculations only accounted for 60% of the effect, but that was promising enough that it seemed the most likely suspect even so.
3. The effect is real, and is caused by some unknown new force. I know several physicists (and know of several more) who took this idea very seriously, and I admit I thought the possibility was kind of cool. One idea is that the Sun’s gravity doesn’t weaken as quickly as predicted by Newton’s law of gravitation (a similar idea to MOND, which I wrote about before). However, if the Sun’s gravity was affecting the Pioneer craft this way, it should also have a similar effect on Pluto and other objects in the far reaches of the Solar System, and this simply wasn’t happening.

The paper announced this week shows pretty convincingly that possibility 2 is correct, accounting for the extra deceleration by calculating how much the heat reflects off the primary antenna—the big dish that is the Pioneers’ most obvious feature. Since that antenna is pointed back toward Earth, any heat emitted would reflect off the dish and act like an additional brake. It’s a perfect example of persistence paying off, along with a clever idea: using graphical techniques developed in the 1970s to find the amount of reflected heat off the dish.

Perhaps we could feel disappointment over this. After all, it would be so much cooler in a way if these venerable spacecraft led us to discover entirely new physics! As romantic as that notion is, however, it’s very rare for such a decisive discovery to happen with such little data. More often new physics is discovered gradually with an accumulation of evidence and creative ideas germinating. So I feel glad that the Pioneer anomaly was resolved, even if it was in a relatively mundane manner; it’s a reinforcement that our science works, and works well.