It began with the search for Planet X. By 1979, Pioneer 10 had accomplished its original mission to become the first Earth-born spacecraft to explore Jupiter and was on its way out of our solar system, flying toward the star Aldebaran — a destination it should reach some two million years from now.

On its way out, Pioneer 10 became a useful partner in an experiment of celestial mechanics. By closely monitoring its trajectory, scientists might detect an unexpected gravitational tug that could betray the existence of the long-hypothesized Planet X. Based largely on unexplained motions in the orbits of Uranus and Neptune, several 20th-century astronomers had suggested the existence of an undiscovered world at the edge of our solar system.

http://planetary.org/news/2005/pioneer_anomaly1_0510.html