A Mercury orbiter offers a unique opportunity to test general relativity and alternative theories of gravity. In 1845, Urbain-Jean-Joseph Le Verrier, a French astronomer, noticed that the point of Mercury's orbit closest to the Sun (perihelion) was moving around the Sun faster than predicted by Newton's theory of gravity. The motion of Mercury remained a mystery until Albert Einstein overhauled the theory of gravity in 1915. BepiColombo will measure Mercury's motion more accurately than ever before and thus provide one of the most rigorous tests ever of Einstein's theory.