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efore the famous physicist came up with his theory of gravity, known as the general theory of relativity, scientists’ predictions for Mercury’s motions were slightly off: The planet’s orbit disagreed with expectations. When Einstein realized that general relativity accounted for the mismatch, it was the first sign his theory was right (SN: 10/17/15, p. 16).Now, physicist Clifford Will has calculated another effect of general relativity on Mercury’s orbit, he reports in a paper accepted in Physical Review Letters. The effect is too subtle to have been detected in observations so far. Planets in the solar system move in elliptical orbits that gradually rotate as each planet journeys around the sun. That rotation is mainly due to gravitational tugs from the other planets. But according to general relativity, gravity is the result of massive objects like the sun and planets warping spacetime. The warping caused by the sun, Einstein realized, would affect the rotation of each planet’s orbit slightly and would be most noticeable for Mercury, since it is closest to the sun. But a smaller quirk of general relativity hadn’t been explicitly calculated until now. The sun’s warping of spacetime also alters how the other planets pull on Mercury, says Will, of the University of Florida in Gainesville. Likewise, the warping caused by the planets changes how the sun pulls on Mercury. The combined effect is so small that it would take 2 billion years to add a degree to the rotation of Mercury’s orbit, Will estimates. Scientists will soon be able to check Will’s calculation. A European and Japanese space mission to Mercury called BepiColombo, scheduled to launch in fall 2018, should be capable of detecting the effect.