Even Einstein, however, may not have got it right. Modern instruments have shown a departure from his predictions, too. In 1990 mission controllers at the Jet Propulsion Laboratory (JPL) in Pasadena, California, which operates America’s unmanned interplanetary space probes, noticed something odd happen to a Jupiter-bound craft, called Galileo. As it was flung around the Earth in what is known as a slingshot manoeuvre (designed to speed it on its way to the outer solar system), Galileo picked up more velocity than expected. Not much. Four millimetres a second, to be precise. But well within the range that can reliably be detected.
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Altogether, John Anderson and his colleagues analysed six slingshots involving five different spacecraft. Their paper on the matter is about to be published in Physical Review Letters. Crucially for the idea that there really is a systematic flaw in the laws of physics as they are understood today, their data can be described by a simple formula. It is therefore possible to predict what should happen on future occasions.

That is what Dr Anderson and his team have now done. They have worked out the exact amount of extra speed that should be observed when they analyse the data from a slingshot last November, which involved a craft called Rosetta. If their prediction is correct, it will confirm that the phenomenon is real and that their formula is capturing its essence. Although the cause would remain unknown, a likely explanation is that something in the laws of gravity needs radical revision.