What is making NASA's twin Pioneer spacecraft mysteriously drift off course, apparently defying the laws of physics? A rigorous new analysis suggests ordinary heat emission can at least partly explain the wayward probes' strange trajectories.
Pioneer 10 and 11 were launched in the early 1970s and explored the outer solar system. But in 1980, mission scientists noticed that the spacecraft have unexpectedly drifted off course.
Both spacecraft have been pulled a little harder than expected towards the sun, and since their launch, they have drifted off course by hundreds of thousands of kilometres.
Possible explanations for this so-called Pioneer anomaly have included technical problems with the software tracking the spacecraft as well as more exotic reasons, such as a breakdown in our understanding of gravity.
But now the verdict is that a substantial part of the anomaly, at least for Pioneer 11, is due to thermal effects, according to Slava Turyshev of NASA's Jet Propulsion Laboratory in Pasadena, California, US. He described his findings on 13 April at a meeting of the American Physical Society in St Louis, Missouri.
For the past two years, Turyshev and his colleagues have analysed telemetry and tracking data for the spacecraft on hundreds of reels of old magnetic tapes and floppy discs to reconstruct the mission in more detail than ever before. They have also examined archived design documents and talked to mission engineers. "It’s like CSI," says Turyshev.
The wealth of data has allowed them to build detailed computer models of Pioneer 11, including a thermal model which shows how heat is distributed over the spacecraft. This has revealed that Pioneer 11 gives off heat in certain directions more than others. The uneven heat emission is enough to nudge the spacecraft off course, accounting for 28% to 36% of the anomaly detected when Pioneer 11 was 3750 million kilometres, or 25 times the Earth-sun distance, away from us.
Turyshev suspects that the optical properties of the spacecraft's exterior may have changed during the mission, possibly degrading due to dust hitting the craft and ultraviolet radiation from the sun.
That might make the spacecraft radiate more heat than expected. However, it's not clear yet whether these changes to the spacecraft's exterior could account entirely for its unexpected trajectory.
Turyshev's team is now planning to test the dust theory and he expects to have results in five months' time.
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