"The Voyager spacecraft has left the solar system," read many headlines over the last few months. But its predecessor, the Pioneer spacecraft, should not be overshadowed. It left an important legacy as well, not for its planetary astronomy discoveries, but for its engineering lessons.
The pathfinding mission for the Voyager spacecraft's tour of the outer planets was the Pioneer mission, with Pioneer 10 launched on March 2, 1972. Just 19 months later, Pioneer 10 passed within 132,000 km of Jupiter, returning more than 500 images taken with its single-pixel camera.
Its primary mission was to explore Jupiter. In fact, Pioneer's discovery of the Jovian radiation belt, 1,000 times as intense as expected, came just in time to add radiation-hardened electronics to the still-under-construction Voyager craft -- electronics that are credited with enabling its long life.
Perhaps the part of Pioneer's mission most important to the engineering community, however, began only after it left Jupiter's influence.
The Pioneer 10 and 11 spacecrafts were the last crafts to have their long range radio dishes aimed back at earth by spin stabilization, rather than adjusted with thrusters or reaction gyro wheels. Between its roughly annual thrust corrections, the Pioneer 10 flew a ballistic trajectory, acting as a test mass to plot out the gravitational fields of the outer solar system.
A group at JPL, lead by John Anderson, had the idea of using the motion of Pioneer 10 as a high-precision probe of the gravitational environment of the solar system, searching for unknown planets and maybe even searching for low-frequency gravitational waves from small wobbles.
After all, this was how Neptune was discovered in 1846. Urbain Le Verrier, astronomer and mathematician, not willing to let go of the accuracy of the Newtonian Theory of Gravity, realized he could account for the well-documented anomalies in the motion of Uranus by postulating a hidden planet in a precise orbit. Neptune was found precisely where he predicted it to make Newtonian gravity work. As François Arago said, Le Verrier was the first astronomer to "find a planet with the point of his pen."
He was not so lucky in applying the same analysis to the motion of Mercury. To account for the well-documented 43 arc-second-per-century anomalous shift in the perihelion of Mercury, Le Verrier proposed the Planet Vulcan, in orbit much closer to the Sun than Mercury. Vulcan was never found, but this anomaly with Mercury was one of the three observations that confirmed general relatively, 70 years later.
This was the context in which Anderson's team set out to do precision celestial mechanics analysis of the motion of the Pioneer spacecraft.