Phobos Overflying Ascraeus Mons. This spectacular picture, taken by Viking Orbiter 2, is the first picture ever taken showing such detail on both a satellite and a primary planet. Phobos is about 22 km across, and Ascraeus Mons is over 300 km across at its base. The complete outline of Phobos is seen from direct and reflected sunlight. Transit pictures such as this are used to determine the size and shape of the satellite as well as improve the map coordinates of features on Mars registered near the satellite's image.
North is to the upper left.