Fenrir was discovered on Dec. 12, 2004, one of 12 Saturnian moons found that day by Scott S. Sheppard, David L. Jewitt and Jan T. Kleyna, using a wide-field camera on the Subaru 8.2-m reflector telescope on Mauna Kea, Hawaii. Brian Marsden computed the orbital elements.
Fenrir has a mean radius of 1.2 miles (2.0 kilometers), assuming an albedo (a measure of how reflective the surface is) of 0.04. It orbits Saturn at an inclination of about 164 degrees and an eccentricity of about 0.1. At a mean distance of 14 million miles (22.5 million kilometers) from Saturn, the moon takes about 1,260 Earth days to complete one orbit.
Fenrir is a member of the Norse group of moons. These "irregular" moons have retrograde orbits around Saturn—traveling around in the opposite direction from the planet's rotation. Fenrir and the other Norse moons also have eccentric orbits, meaning they are more elongated than circular.
Like Saturn's other irregular moons, Fenrir is thought to be an object that was captured by Saturn's gravity, rather than having accreted from the dusty disk that surrounded the newly formed planet as the regular moons are thought to have done.
How Fenrir Got its Name
Originally called S/2004 S16, Fenrir was named for a monstrous wolf in Norse mythology who was the offspring of Loki, the disgrace of the gods, and Angrboda, a disagreeable giantess. The gods managed to bind Fenrir using a dwarf-manufactured fetter made of the sound of a cat's footfall, a woman's beard and other hard-to-find components. According to the mythology, Fenrir is destined to break free at doomsday (the time known as Ragnarok) and kill Odin, the supreme Norse god.