The 'Pioneer anomaly' – the mystifying observation that NASA's two Pioneer spacecraft have drifted far off their expected paths – cannot be explained by tinkering with the law of gravity, a new study concludes.

The study's author suggests an unknown, but conventional, force is instead acting on the spacecraft. But others say even more radical changes to the laws of physics could explain the phenomenon.

Launched in the early 1970s, NASA's Pioneer 10 and 11 spacecraft are drifting out of the solar system in opposite directions, gradually slowing down as the Sun's gravity pulls back on them.

But they are slowing down slightly more than expected and no one knows why. Some physicists say the law of gravity itself needs revising, so that gravity retains more strength in the outer solar system. But there has been disagreement about whether such modifications would accurately predict the orbits of the outer planets.

Now, Kjell Tangen, a physicist at the firm DNV in Hovik, Norway, says tweaking the law of gravity in a variety of ways cannot explain the anomaly – while also getting the orbits of the outer planets right. After modifying gravity in ways that would match the Pioneer anomaly, he inevitably got wrong answers for the motion of Uranus and Pluto.

Conventional explanation

That suggests conventional physics – such as drag due to dust grains in space, or the emission of heat from small nuclear generators on board, known as RTGs, in some directions more than others – probably causes the anomaly, Tangen says. But he admits a definitive cause remains elusive. "It is easier to be conclusive about what cannot be the cause," he told New Scientist.

Myles Standish, who calculates solar system motions at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, says most scientists suspect the asymmetrical radiation of heat from the spacecraft is to blame.

But he also acknowledges that the orbits of Uranus, Neptune and Pluto have not been measured as precisely as those of the inner planets, suggesting the new study by Tangen cannot rule out modified gravity as a cause. "The measurements are not able to support any definite conclusions," he told New Scientist.

Extreme changes

Other scientists say the effect could be explained by even more extreme changes to Einstein's general theory of relativity, since Tangen did not alter one of its central tenets – the equivalence principle.

The principle says that all objects respond to gravity in the same way regardless of their mass, composition or the paths they took to their present location. Among other things, it explains why a feather and a bowling ball fall at the same rate in a vacuum.

If you allow violations of the equivalence principle, modifying the laws of physics can explain the Pioneer anomaly without messing up the orbits of the outer planets, says Robert Sanders of the University of Groningen in the Netherlands.

A theory called modified inertia, proposed by Mordehai Milgrom of the Weizmann Institute of Science in Rehovot, Israel, does just this. It says the way objects accelerate under gravity depends on their past trajectories – a breach of the equivalence principle. In this scenario, the Pioneer spacecraft, whose trajectories are taking them out of the solar system, experience an anomaly, while the outer planets, whose orbits keep them bound to the Sun, do not.

Re-flying the mission

If confirmed, Tangen's conclusions would be very significant, Sanders says. "Either the Pioneer anomaly isn't real – that is, it's another physical effect that they haven't taken into account, or whatever modification of gravity it is doesn't obey the equivalence principle," he told New Scientist.

That confirmation could come relatively quickly. Slava Turyshev of NASA has been compiling additional data from the Pioneer 10 and 11 spacecraft that had been unavailable because they were in archaic file formats and storage media.

The data holds information about the spacecraft's internal behaviour, including the heat released by the RTGs. This can be compared to the tracking data to see whether the Pioneer anomaly matches the changes in heat radiated throughout the spacecraft's lifetime.

The analysis is "going reasonably well", Turyshev told New Scientist. "We should be able to tell more on the anomaly in a year or so."

Journal reference: Physical Review D (in press)

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