Voyager 1 image of Jupiter's moon Callisto from a distance of 350,000 km. Callisto is about 4,800 km in diameter (about the size of Mercury). Features as small as 7 km across can be seen in this image. The large "bulls-eye" at the top of the image is believed to be an impact basin formed early in Callisto's history. The bright center of the basin is about 600 km across and the outer ring is about 2600 km across.