Astronomy
The Pioneer Anomaly: Historical Puzzles in Spacecraft Navigation
Quick fact
When the Pioneer 11 anomaly was first noticed in 1980, it appeared to be a constant acceleration pulling the spacecraft back toward the Sun at about 8.74 × 10⁻¹⁰ m/s²—but after a decade of analysis, the cause was traced to heat leaking from the spacecraft's own power systems.
Why this is interesting
NASA's Pioneer 10 and 11 spacecraft were mysteriously slowing down as they hurtled toward interstellar space. Could it be an unknown force from deep space? Or something far more mundane?