Hydrogen on Vesta. Satellite image of the asteroid Vesta (4 Vesta), showing areas around the equator that are abundant with hydrogen, from red (high) to grey (low). Vesta orbits the Sun between the orbits of Mars and Jupiter at an average distance of 353 million kilometres. It is the second largest asteroid, having a diameter of about 538 kilometres. Under favourable conditions, it is the only asteroid visible to the naked eye. The hydrogen probably exists in the form of hydroxyl or water bound to minerals in Vesta's surface. Image composed of data from the gamma ray and neutron detector (GRaND) on NASA's Dawn spacecraft collected from December 2011 to May 2012, superimposed on a mosaic of images and a 3-D model created from data obtained by Dawn's