60-Second Space

Pioneer Spacecraft Warmth Takes Heat off Relativity

The tiny slowing of the two Pioneer spacecraft, known as the Pioneer anomaly and considered by some to challenge general relativity, is probably due to the heat produced by electronics and radioactive decay. John Matson reports

Challenge Einstein, and you’d better be prepared to lose. 

There was the recent case of the neutrinos that now appear not to exceed the speed of light. And now a puzzling quirk in the trajectories of two 1970s-era spacecraft appears not to challenge what we know about physics.  

The Pioneer 10 and Pioneer 11 probes have been traversing the solar system for some 40 years. But as they venture outward, they’ve been slowing down a bit more than would be expected from the sun’s gravitational pull.

It was thought that the discrepancy, known as the Pioneer anomaly, could reveal a flaw in Einstein’s general relativity, the reigning theory of gravity. But a new analysis offers a more mundane explanation.

Pioneer 10 and 11 are slowing down due to a small-but-ever-present thermal recoil. Both spacecraft give off heat from their electronics and from the radioactive decay of their plutonium fuel, and that’s enough to impart the measured deceleration. The researchers liken it to photons from a car’s headlights pushing gently back on the vehicle. The analysis is in the journal Physical Review Letters. [Slava G. Turyshev et al., "Support for the Thermal Origin of the Pioneer Anomaly"]

So the Pioneer anomaly probably does not mean general relativity is broken. Score another one for Einstein.

—John Matson

[The above text is a transcript of this podcast.]

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