Our understanding of gravitational dynamics is very good. With it we can send probes to distant worlds such as Pluto to an accuracy of a dozen kilometers. But sometimes our models don’t quite match reality. These astrometric anomalies are small and subtle, but they sometimes lead to a new understanding of the universe.
Ten Billionths of a Gee
Pioneer 10 was launched in 1972. The next year Pioneer 11 was launched. Their mission was to fly past Jupiter and then Saturn, making the first detailed studies of the planets. Afterwards, they continued their journey to the outer reaches of the solar system. At this point they entered a second phase of their mission, to study the diffuse gases in the solar system on their way to interstellar space. Their observations indicated that something rather strange was going on.