Credit: NASA

In 1998, scientists poring over trajectory data from the Pioneer 10 (pictured) and 11 spacecraft found an unexplained acceleration acting on the two vessels, which were launched by NASA in the early 1970s to observe Jupiter. This led theorists to invoke exotic new theories of gravity.

But by rescuing and analysing additional data from decades-old archives, Slava Turyshev at NASA's Jet Propulsion Laboratory in Pasadena, California, and his colleagues say that standard physics can explain what is known as the 'Pioneer anomaly'. The authors boosted the number of tracking points for the spacecraft from 30,671 to 122,591 and found that the acceleration, instead of being constant, was actually slowly decreasing with time. This would be consistent with the emission of heat from a degrading radioisotope power source on board each probe, which would apply a tiny amount of force to the spacecraft.

Phys. Rev. Lett. 107, 081103 (2011)