The Pioneer anomaly: an unexplained force, and how it was explained

Pioneer 10 and Pioneer 11 left Earth in 1972 and 1973, crossed the asteroid belt, passed Jupiter and Saturn, and headed out of the solar system. Their trajectories were tracked by radio for decades with extraordinary precision.

By the 1980s the tracking showed something wrong. Both were decelerating slightly more than gravity accounted for — a tiny extra pull towards the Sun, around eight ten-billionths of a metre per second squared.

Why it was taken seriously

Because it was small, consistent and measured on two independent spacecraft.

An error in one probe’s tracking would be a fault. The same anomaly, of the same magnitude, in the same direction, on two vehicles launched a year apart and heading in different directions, is a signal.

And it persisted through every recalculation. Gravitational effects of known bodies, relativistic corrections, solar wind, drag from interplanetary dust, errors in the ephemeris, clock effects — checked, and none accounted for it.

The exotic proposals

It attracted them, as such things do. Modified Newtonian dynamics, which alters gravity at very low accelerations and was devised for galaxy rotation curves, predicted deviations in roughly the right regime. Extra dimensions were proposed. Dark matter distributions were suggested. So was a variation in the gravitational constant.

These were serious papers by serious people. If the anomaly had held, it would have been the most important result in gravitational physics in a century.

What it turned out to be

Heat, radiated unevenly.

The spacecraft carry radioisotope thermoelectric generators and electronics that produce heat, which is radiated away as infrared. Photons carry momentum, so radiating more heat in one direction than another produces a small thrust.

Establishing this required recovering the original telemetry — much of it on decades-old media — and building a detailed thermal model of the spacecraft, tracking how heat flowed and where it left. The work took years. The answer accounted for the anomaly.

Why this is the most useful piece in the section

Because it is a complete cycle. A real anomaly, taken seriously, attracting exotic explanations from credentialled people, resolved by recovering the primary data and doing the difficult unglamorous modelling — and the answer was ordinary physics that nobody had bothered to calculate.

Every unexplained case on this site could end this way, and the ones that do will end like this: not with a revelation, but with somebody finding the tapes.