The Pioneer 10 and 11 probes are currently heading out of the solar system, but they're not quite doing it quickly enough. This physics-defying anomaly has stubbornly defied explanation, but an old computer graphics technique has finally solved the conundrum.
The Pioneer probes are both on escape trajectories that will eventually take them out of the solar system. They're travelling fast, but both are slightly decelerating because the Sun's gravity is pulling them back. The so-called Pioneer Anomaly comes from the fact that both probes are slowing down slightly more than they ought to. It's less than an extra billionth of a meter per second squared, but that's still enough to fall outside our understanding of physics.
There was much speculation on the sorts strange and bizarre hidden effects that could be causing this, including the exotic idea that gravity itself somehow becomes stronger over the distances separating the Sun from the Pioneer probes. These by and large fell by the wayside when physicists realized the heat produced by the probes might be able to account for the extra deceleration. But even then, calculations revealed thermal effects could only account for about two-thirds of the anomaly, still leaving the basic mystery unsolved.
That's where researchers at Portugal's Institute for Plasmas and Nuclear Fusion enter the picture. They realized that all the previous calculations had only looked at the heat emitted, ignoring any heat reflected back at the probes. They used a computer modeling technique first developed in the 1970s known as Phong shading to figure out how the heat would reflect off the spacecraft and in which direction it would then travel.
Phong shading, which is often used to render the reflection of light off of 3D objects, proved just as adept at working out the various heat reflections on the Pioneer craft. The technique revealed that heat emitted from the main equipment compartment bounces off the back of the probes' antennae. These antennae are of course pointed back towards Earth to allow information to be relayed, which means they're also pointed at the Sun. In turn, any heat reflected off the back of these antennae will create a force in the direction of the Sun.
With this secondary effect taken into account, the Portuguese researchers are able to completely able to remove the pesky anomaly, as they explain in their paper:
"With the results presented here it becomes increasingly apparent that, unless new data arises, the puzzle of the anomalous acceleration of the Pioneer probes can ﬁnally be put to rest."
Of course, this won't be considered generally accepted until other groups are able to confirm and replicate these results. But this is a convincing result, and it seems far, far more likely for this to be the case than some otherwise undiscovered property of physics. Hopefully, the Pioneer Anomaly can soon be put to rest, and we can get back to what's really amazing about all this - that both these probes, sooner or later, will actually leave our solar system behind and become, along with the Voyager probes, humanity's first interstellar travelers.