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Exotic cause of 'Pioneer anomaly' in doubt (David Shiga, 6/22/07, New Scientist)

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.