The ratio of magnesium to iron in an igneous rock, which is related to the chemical composition of the mantle material. As the magma cools, lighter minerals solidify first, forming a scum on the surface of the molten rock. If the composition of the mantle is known, the magnesium number can be predicted, and the magnesium number provides information on the composition of the mantle rock. The magnesium number also provides information on the thermal history of the rock. One task for the SMART-1 (Small Mission for Advanced Research in Technology) mission to the Moon, which was deliberately crashed into the lunar surface on 3 September 2006, was to determine the magnesium number of lunar surface rocks. If, as expected, the lunar magnesium number is lower than that of rocks on Earth it will support the view that the Moon was blasted from the Earth by an impact and the surface rocks on both bodies are derived from the same mantle material. The lower lunar value would reflect the different thermal histories of the Earth and Moon. If the lunar magnesium number is the same as that on Earth, however, it would prove that the two bodies could not have originated from the same source.