Pioneer 10’s puzzling motion: a lot of hot air

ST. LOUIS — For years, astronomers have been puzzled about why the Pioneer 10 and 11 spacecraft aren’t traveling quite as fast as they ought to be. Launched in the early 1970s, the craft have each journeyed several billion kilometers in opposite directions from Earth and are now at the edge of the solar system. But each year, the craft have lagged behind their expected travel distance by about 5,000 km.

Researchers have been mulling over the discrepancy for more than a decade. Some have proposed that a new type of physics, perhaps a modification of general relativity, is necessary to explain why the crafts’ speeds don’t match that predicted by the standard theory of gravity.

A new analysis of the thermal properties of Pioneer 10, using data not available six years ago, reveals a mundane explanation for a substantial part of the anomaly in the craft’s speed. Because Pioneer 10 radiated heat more strongly in one direction than others, it didn’t leave the inner solar system quite as fast as expected, says Slava Turyshev of NASA’s Jet Propulsion Laboratory in Pasadena, Calif. When the craft was 25 times as far from the sun as Earth is, Turyshev calculates, about 30 percent of the unexplained slow down in velocity can be traced to the uneven way that the craft radiated heat produced by its plutonium-powered thermoelectric generator. He reported the findings April 12 in St. Louis at a meeting of the American Physical Society.

Turyshev is now analyzing how much of an effect the heating had on the craft’s velocity when Pioneer 10 was farther from the sun and more of its power source, plutonium-238, had decayed away. He also plans a similar analysis of Pioneer 11, which also has a plutonium-powered generator.

Until then, the jury is still out on whether the remaining 70 percent of the discrepancy in Pioneer 10’s velocity requires some more exotic explanation.

“My guess is [the discrepancy] will go away,” predicts physicist Eric Adelberger of the University of Washington in Seattle, who specializes in high-precision tests of gravity. “I just don’t think that spacecraft are a very good way to learn about gravity, because they’re so light and small” and can easily be pushed around by a multitude of non-gravitational effects, he says.

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