Phong Shading and the Pioneer Anomaly

Have you heard of the Pioneer anomaly? Pioneer 10 and Pioneer 11 are two spacecraft which were launched into the outer solar system back in the early 70’s. They’re now about 20 ALUs away and are on their way out of the solar system. The anomaly is that their velocity doesn’t quite match predictions.

There’ve been a number of interesting theories proposed to explain this anomaly. Some of these theories involve things like dark matter or modifications to our current theory of gravity.

Now some researchers in Portugal have a new theory. It’s a bit more prosaic. It involves a more refined calculation for how thermal radiation modifies the speed of the spacecraft. You can find their paper here. To get this right, they needed to calculate how the thermal radiation reflects off the different parts of the spacecraft. To do this, they used Phong’s reflection model. That makes sense. It’s a good approximation when you’re computing lighting, and thermal radiation is just photons of a different color.

It’s kind of appropriate that they used a computer graphics technique, since the Pioneer mission has long been a favorite in the graphics community because of the flyby animation that Jim Blinn did for JPL many years ago.