A Ghost in the Machine
Launched in 1972 and 1973, the Pioneer 10 and 11 spacecraft were humanity's first envoys to the outer solar system. After completing their primary missions at Jupiter and Saturn, they continued their flight into deep space. By 1980, however, navigators at NASA's Jet Propulsion Laboratory noticed something odd. After the spacecraft passed a distance of 20 astronomical units (AU) from the Sun, both began to slow down more than predicted by the laws of gravity.
The effect was incredibly small. The spacecraft were experiencing a constant, unmodeled acceleration directed back toward the Sun with a magnitude of (8.74 ± 1.33) × 10⁻¹⁰ m/s². This deceleration is equivalent to reducing the spacecrafts' outbound velocity by just 1 kilometer per hour over a decade. For years, this tiny discrepancy, known as the Pioneer Anomaly, was a deep mystery. Proposed explanations ranged from the gravitational pull of unseen matter to a fundamental flaw in Einstein's theory of general relativity. The precision of the spacecrafts' tracking data was so high—thanks to their spin-stabilized design—that this minuscule effect was not a as a measurement error.
The Glow of an Answer
The solution to the puzzle was to be more terrestrial in origin, though it took decades of painstaking work to confirm. A team of scientists led by Slava Turyshev at JPL initiated a comprehensive data recovery effort, sifting through old magnetic tapes and telemetry data to build a complete thermal model of the spacecraft. The cause was heat.
The Pioneer probes are powered by radioisotope thermoelectric generators (RTGs), which use the decay of plutonium-238 to generate electricity. This process also produces a significant amount of waste heat. In the vacuum of space, this heat can only escape as thermal radiation. The scientists discovered that the design of the spacecraft resulted in an anisotropic, or uneven, radiation of this heat. More heat radiated from the surfaces of the RTGs and the main equipment box that faced away from the sun. This imbalance in radiated thermal photons created a tiny but persistent recoil force, a "photon rocket" effect, pushing the spacecraft back toward the sun. The calculated force from this thermal recoil almost perfectly matched the observed anomalous acceleration, resolving the mystery.