The Pioneer Anomaly
Later explainedSummary
Two spacecraft leaving the solar system were slowing down slightly more than gravity said they should. The excess was almost unimaginably small: about eight parts in ten billion of the acceleration you feel standing on the Earth. At that rate it would take a hundred thousand years to get something up to walking pace.
But the tracking was good enough to see it, and it would not go away, and for thirty years it was one of the best reasons anyone had to think there might be something wrong with our theory of gravity.
There was nothing wrong with gravity. The spacecraft were being pushed by their own waste heat.
What is documented
The spacecraft. Pioneer 10, launched in 1972, and Pioneer 11, launched in 1973. The first probes to cross the asteroid belt, the first to fly past Jupiter, and the first objects built by humans on trajectories out of the solar system.
The measurement. Their positions were tracked by Doppler radio, and the precision of that tracking is the reason this record exists. From around 1980, the data showed both spacecraft experiencing a small additional acceleration directed back toward the Sun, over and above everything that could be accounted for.
The size of it. Roughly 8.74 × 10⁻¹⁰ metres per second squared. To state that in a way a person can hold: it is about ten billion times weaker than the pull you feel standing on the ground.
It was real. This is the part that made it serious. The effect showed up on both spacecraft. It was not a modelling artefact, it did not go away when the analysis was redone, and it survived thirty years of people trying to make it go away. It was measured, repeatedly, by people who very much wanted to find a mundane cause.
What was proposed. Because the anomaly was a deviation from predicted gravitational behaviour at large distances, it drew serious attention from physicists. Proposals included modifications to Newtonian dynamics, effects of dark matter, and various forms of new physics. None of this was crankery. A genuine, persistent, unexplained deviation from gravitational prediction is exactly the sort of thing that ought to be taken seriously, and it was.
The data was nearly lost. Decades of Doppler tracking data and spacecraft telemetry existed only on ageing magnetic tapes and in boxes of records, much of it uncatalogued and deteriorating. A recovery effort, involving JPL and supported in part by the Planetary Society and by volunteers, retrieved and digitised it.
That effort is the reason the anomaly could be solved, and it is worth saying plainly: somebody went into a storage room and rescued the answer from a decaying tape.
Leading explanations
Anisotropic thermal radiation. Established, 2012.
Everything on those spacecraft that produces heat radiates that heat away as infrared photons. Photons carry momentum. If a spacecraft radiates more heat from one side than the other, it feels a push in the opposite direction. It is a rocket engine of unimaginable feebleness, running on waste heat, and it never switches off.
Pioneer 10 and 11 were powered by radioisotope thermoelectric generators, which turn the heat of decaying plutonium into electricity. They are extremely hot and extremely inefficient, which means most of that heat is simply dumped. The spacecraft also carried a large dish antenna, which acted as a reflector.
The net result was that the craft radiated slightly more heat in one direction than the other, and were therefore very gently pushed the other way.
How it was proved. Slava Turyshev and colleagues at JPL built a full thermal model of the spacecraft, reconstructing them from the original engineering drawings and from the recovered telemetry, and computed the recoil force. It matched.
And here is the fingerprint. The plutonium in the generators decays. As it decays, it produces less heat. If the anomaly is thermal, it must decay along with the plutonium.
It does. The recovered data shows the anomalous acceleration falling off over the decades in step with the decay of the power source. That is not a fit. That is a signature, and it is the reason the case is closed.
The result was published in Physical Review Letters in 2012.
What the popular version gets wrong
"Scientists thought it was new physics and they were being silly." They were not. A persistent, reproducible, unexplained deviation from gravitational prediction, seen on two independent spacecraft, is precisely what would herald new physics if new physics were there. Taking it seriously was correct. The people who took it most seriously are the people who eventually solved it.
"It was obviously just heat." It was not obviously anything. Modelling the thermal emission of a spacecraft you cannot see, decades after it was built, using drawings and telemetry from the 1970s, is very hard, and it is why the answer took thirty years rather than three.
"The anomaly proved gravity is wrong." It proved that a spacecraft leaking heat is a very small rocket.
"It was a waste of thirty years." It produced an unusually careful piece of spacecraft thermal modelling, it forced a serious re-examination of gravitational tracking at solar system scales, and it demonstrated that a real anomaly can be pursued to a clean resolution without anybody having to invoke anything exotic. That is what the process is supposed to look like.
Current status
Later explained. Mechanism identified, magnitude computed, and confirmed by the decay signature that follows the plutonium.
The registry keeps this record as a model of how an anomaly should be handled, and it is the calmest story in the archive.
Nobody covered anything up. Nobody was ridiculed. The effect was real, it was reported, it was pursued for thirty years by people who kept looking for a boring explanation, and eventually somebody found the boring explanation, which turned out to be that two of the loneliest objects ever made were being pushed on, imperceptibly, by their own warmth.
Sources
- Turyshev, S. G., Toth, V. T., Kinsella, G., Lee, S.-C., Lok, S. M. and Ellis, J. (2012). "Support for the Thermal Origin of the Pioneer Anomaly." Physical Review Letters 108, 241101.
- Anderson, J. D. et al. (1998, 2002). Initial reports and characterisation of the anomalous acceleration.
- Recovered Pioneer 10 and 11 Doppler tracking data and spacecraft telemetry, JPL, with support from the Planetary Society.
- Thermal reconstruction of the spacecraft from original engineering drawings.
Last reviewed: July 2026. Records are provisional. Where the evidence changes, the entry changes. Found an error? Tell us.