Aristarchus crater on the Moon, Hubble Space Telescope image. The crater is the white area at lower right. It is 42 kilometres in diameter, and is thought to have formed between 100 and 900 million years ago. The valley-like structure at upper centre is the Schroter's Valley rille. This image was taken on 21 August 2005 by Hubble's Advanced Camera for Surveys instrument, during a survey that used ultraviolet and optical imaging to search for lunar mineral deposits.