Pioneer 11 launched

On this day in 1973 NASA launched its Pioneer 11 probe on a mission to explore the Solar System beyond the orbit of Mars. By November 1974, Pioneer 11 had arrived at Jupiter, where it provided the first detailed look at the planet’s atmosphere. Five years later, the probe became the first built by humans to reach Saturn. At its closest approach, Pioneer 11 flew through the gas giant’s rings, narrowly missing one of its small moons (most likely Epimetheus). NASA continued to track Pioneer 11 until 1995, when contact was lost. A small discrepancy between the probe’s expected range (based on orbital mechanics) and its measured range (determined by radar) became known as the Pioneer anomaly. Although known and unknown effects were invoked to account for the anomaly, the most plausible explanation remains asymmetric thermal heating, which causes the probe’s shadowed side to exert less radiation pressure than its sunlit side.

Date in History: 6 April 1973