Pioneers 10 and 11 were launched in 1972 and 1973 to study the outer solar system. In the late 70's small deviations of their trajectory were used to try to spot new planets. Instead of finding small forces pointing to the outer solar system it was found a force pointing to the inner solar system as if the gravity from the sun were a little bit stronger. This is the Pioneer anomaly. Many people think the drift is due to a malfunction of the spacecrafts or due to some trouble with the tracking stations on Earth. Others think some new physics may be at work.
What I did not know was that the data which gave rise to this finding was collected from a small part of the trajectory. The Pioneers have been flying for more than 30 years now but the anomaly data cover 11.5 years for Pioneer 10 and 3.75 years for Pioneer 11. A new article in New Scientist Have we got gravity all wrong? (unfortunately you need a subscription) tells the full story. Now people are collecting the old magnetic tapes and floppy discs containing all the 30 years of telemetry data to try to reconstruct the full space mission!
If the anomaly is real there is a proposal to launch a dedicated spacecraft to measure the effect between 2015 and 2025. Meanwhile it is better to keep a close eye on the trajectories of the next satellites going to the outer solar system.