Pioneer 10, launched in 1972, and Pioneer 11, launched a year later, were originally designed to study Saturn and Jupiter. They have long since passed the two gas giants and become the first man-made objects to leave our solar system.

Radio contact has been lost, but know where they are, and where they are isn't where they should be. According to calculations based on Newton's Laws, they are drifting ever-so-imperceptibly off course. This meant one of two things - either there was a flaw in the spacecraft, or Newton had got something wrong.
Since the Pioneer Anomaly was first identified in the early Eighties, space wonks have been working furiously to figure out what the problem is. They have pored over the numbers, taken the Voyager's design to bits and pulled together all the data they can find (including the very earliest flight logs, which were printed out on paper and left in a stairwell, where they were rescued hours before they were due to go in a skip).
They quickly established that the Voyager craft were off course because they were slowing down too fast. They were meant to slow down anyway, due to the gravitational effect of the Sun, but there was something else putting the brakes on. Then some bright spark noticed that the rate of deceleration was exponential and matched the decay rate of plutonium-238.
This is significant, because plutonium powers both Voyagers. Science wonks now believe that a heat differential of as little as 5% between the front and rear of the craft would be enough to produce the observed deviation. I know it's been bothering you and wanted to be the first to put your mind at rest.