Pioneer 10 should not have changed course at all. After flying past the giant planet Jupiter, it left the solar system at a speed of 37 kilometers a second, heading for the star Aldebaran. It flew through empty interplanetary space, where there is no friction and its trajectory was determined by the gravitational pull of the Sun and the larger planets. The Pioneer Project’s flight controllers could predict the position of the small space vehicle many years ahead. But Pioneer 10 did not keep to its designated route. In December 1992, it changed course very slightly, as if something had given it the lightest of taps.