Pioneer 10 and 11 are spacecrafts launched in 1972 and 1973. You may remember that they include a plaque that shows nude human figures for the titillation of any aliens who may discover them. Physicists have been worried about the fact that they are not moving quite as fast as one would expect (by about 300 miles per year). This has led to speculation that there might be something wrong with general relativity. Even though the extra acceleration amounted to only about 8.7 x 10-10 m/s, it was an anomaly that required some kind of an explanation. I learned from an article by Dennis Overbye in the NYT on Tuesday, July 24, that a wonderful resolution to the problem has been provided. The answer, by Turyshev, Toth, Kinsella, Lee, Lok, and Ellis of the Caltech Jet Propulsion Laboratory was published in the June 15 issue of Physical Review Letters, is that the heat provided by the decay of a plutonium-238 to power the electronics is radiated anisotropically because the object maintains a constant attitude relative to the sun. The infrared photons that are radiated because the object is warmer than its surroundings also have momentum (remember the deBroglie equation!). As Overbye puts it, Pioneer has been driving with its high beams on! When the Pioneers were launched, the momentum of the hot rocket fuel was directed down, resulting in the equal and opposite acceleration of the rocket upward. In outer space, the forward momentum of the photons causes the spacecraft to slow for basically the same reason. I will certainly use this example when I next teach about the momenta of photons in my quantum chemistry course.