The team looked at every known aspect of the probe’s design but just couldn’t find anything that would explain the deceleration. Things really got weird when they looked at Pioneer 11, a sister ship headed in the opposite direction, and then at the Ulysses probe, currently circling the sun. They, too, seemed to be experiencing unexplained tug in the direction of the sun. Eventually Anderson’s team had to face an unsettling possibility: with no obvious cause in sight, possible explanations dwindled to a fundamental problem with our understanding of gravity or time. The idea that space probes are being affected by unknown physics is almost as disturbing to astronomers as the thought of intergalactic marauders would be to the rest of us. “People don’t want to believe that there’s something wrong [with our understanding of physics] because that messes everything up,” says Michael Nieto, a physicist at Los Alamos National Lab who worked with Anderson on the analysis. While he believes a more prosaic explanation will eventually be found, Anderson says that “after pursuing an answer for 20 years and not finding it, we felt it was important to get it out for discussion.”
When they published their observations in the journal Physical Review Letters, discussion is exactly what Anderson’s team got. “When you hear that something’s wrong with gravity you get pretty interested,” says Edward Murphy, an astronomer at Johns Hopkins University who published a possible explanation in a recent issue of the same journal. Anderson’s team noted that in the near-vacuum, low-gravity environment of space, the small amount of heat radiating out from the probes’ electrical systems might provide enough push to account for the speed change. But then they calculated that heat would dissipate equally in all directions with no net effect. Murphy, who has worked on other probes, says they missed a design detail; heat vents are located on the side facing away from the sun, so they don’t heat up in the solar radiation. Murphy says his calculations show that the slight, constant nudge of heat from the vent could provide “half or more of the total explanation.” Jonathan Katz, an astronomer at Washington University in St. Louis, thinks he’s worked out where the rest of the push is coming from. The probes rely on small plutonium-powered nuclear reactors called RTGs for their electricity. Katz calculates that some of the excess heat given off by the RTGs would bounce off the back of the probes’ radio antennas, providing enough of a shove toward the sun to explain the deceleration. Interesting ideas, says Anderson, but “our estimate is that they fail by a factor of five or more.”
Analysis of the discrepancy is already helping engineers perfect navigation in space. But a definitive solution to the mystery may have to wait until a new probe heads for the far reaches of the solar system. The next opportunity will likely be the Pluto-Kuiper Express, slated to reach Pluto sometime after 2010. But what if new information doesn’t explain the phenomenon? “Then you have to look at the theory,” says Nieto. It could be one of two things. “Either gravity is different than we think it is or time is messed up somehow.” Those prospects are exactly what the physics community is resisting. “General relativity has been phenomenally accurate in predicting motions of planets, which we’ve been tracking for hundreds of years,” says Murphy. If there were something wrong with the theory, “we would have seen it by now.” Unless there turned out to be a small difference in the way gravity affects large objects like planets and small ones like probes. But that would contravene the principle of equivalence, one of the central tenets of physics. Simply put, gravity is supposed to affect all things equally; that’s why pebbles and boulders fall at the same rate in a vacuum. “If you’re going to violate such a strong principle you need much stronger evidence than this,” says Murphy. That means Isaac Newton doesn’t have to worry about his reputation just yet.
