Enceladus is the sixth largest moon of Saturn and the brightest moon in the solar system. It was discovered by William Herschel in 1789. Before the Voyager probe of Saturn in the 1980s, Enceladus was known only as an ice-covered moon. Voyager revealed that Enceladus is about 500 kilometers across (about one-tenth the diameter of Saturn's largest moon, Titan) and that its surface reflects almost 100 percent of sunlight. Voyager 1 found that Enceladus's orbit is in the densest part of Saturn's E ring, suggesting that there may be some connection between the two; Voyager 2 found that, despite its small size, the moon has both ancient craters on its surface and younger, geologically distorted topographic formations - some of which are as young as 100 million years old.