The puzzle is that Pioneer 10 is slowing more quickly than it should.
It was initially suggested that this might be due to the force from a tiny gas leak or that it was being pulled off course by the gravity of an unseen Solar System object.
The mystery deepened further when an analysis of the trajectory being followed by its sister spacecraft, Pioneer 11, launched in 1973, showed that it too was being subjected to the same mysterious effect.
But Pioneer 11 is on the opposite side of the Solar System from Pioneer 10, about 22 billion km (about 14 billion miles) away. This means the effect cannot be the gravitational effect of some unseen body.
But others have dismissed this as being too fanciful, arguing that if the Pioneer anomaly was really indicative of a change in our understanding of gravity, then it would be apparent in the orbits of the planets around the Sun – which it is not.