Pioneer anomaly

Settled in 2012 by Slava G. Turyshev and colleagues (thermal recoil force modeling).

Precise tracking of the Pioneer 10 and 11 spacecraft from the mid-1980s onward revealed a small, consistent sunward deceleration unaccounted for by known gravitational effects, sparking speculation about modified gravity or new physics for over two decades. In a detailed 2012 study, Turyshev and colleagues built a finite-element thermal model of each spacecraft using its full flight telemetry and showed the anomaly matched the recoil force from heat (generated by the radioisotope power source and onboard electronics) radiating anisotropically off the spacecraft body, concluding no anomalous acceleration remained once this thermal recoil force was properly accounted for. The explanation has since been supported by independent groups using different computational techniques.

A mystery that briefly threatened to rewrite the law of gravity across billions of kilometers of empty space turned out to be explained by waste heat quietly leaking off the spacecraft's own electronics.

Explanations that circulated

• That the Pioneer anomaly was evidence of a breakdown in Newtonian gravity or general relativity at large distances (a popular speculation through the 2000s) — it was instead fully explained by mundane, if subtle, spacecraft engineering effects

References

• Support for the Thermal Origin of the Pioneer Anomaly, S. G. Turyshev, V. T. Toth, G. Kinsella, S.-C. Lee, S. M. Lok, J. Ellis, Physical Review Letters, 2012