Credit: NASA
Slava Turyshev of JPL's Relativistic Astrophysics research group agrees. "Before you report that you have discovered a wonderful new physics, you have to do your homework," he says.
That homework involves weeding out mundane explanations, one by one. In some cases, it's easy. A gas leak, for example, shouldn't have continued for years. "I don't think anybody's suggesting gas leaks any more," Anderson says.
Another simple prospect was that the force relates to solar radiation pressure. When sunlight hits an object, it imparts very slight momentum to it, like wind on a sail. It's a big enough effect that theoretical physicists and science fiction writers have sketched plans for 'solar sail' spacecraft that could take advantage of the effect for propulsion.
The strength of the pressure depends on the reflectivity of the surfaces being hit by sunlight. Brighter surfaces get more impulse, so if something after launch altered the spacecrafts' sheen, the radiation pressure might have changed by enough to account for the anomaly.
Radiation pressure, however, falls off with the intensity of sunlight, which drops with the square of the distance from the Sun.
The Pioneer probes were tracked long enough before NASA finally lost contact with one, in 1995, and the other, in 2003, for the scientists to determine that the anomaly doesn't follow this formula. So much for that idea.
As for the possibility that we need to revise our understanding of physics, perhaps dark matter – an unknown substance whose existence is inferred from its gravitational effect on galaxies – is somehow retarding the spacecraft as they zoom toward interstellar space.
But that would imply that there is dark matter within the Solar System, and nobody's ever seen any other evidence of that.
As alternative theories fall by the wayside, it's tempting to wonder whether it really is new physics.
Perhaps our understanding of gravity is off by a tiny amount. Or perhaps something involving gravity or velocity altered time on the two spacecraft, causing the signal drift.
Such effects indeed occur under Einstein's theory of general relativity, but all known ones have been taken into account. "You might have to modify general relativity to include the Pioneer anomaly," Anderson says.
Adding credence to this concept is the observation that the mystery force appears to be very close to the product of two fundamental constants: the speed of light and the Hubble constant, which is related to the rate at which the universe is expanding.
Specifically, the Pioneer anomaly, for both spacecraft, is an acceleration of 8.7 +/- 1.3 x 10-10 m/sec2. The product of the speed of light and the Hubble constant is about 6.7 in the same units. "Some people think that's significant," Anderson says.
