Fenrir is a member of the Norse group of moons, which orbit Saturn at mean distances ranging from 12 to 24 million km, at inclinations between 136 and 176 degrees from the plane of Saturn's equator, and with eccentricities between 0.12 and 0.77. (A satellite's eccentricity is a number between 0 and 1 which describes the shape of the orbit. The closer to 0, the more circular it is; the closer to 1, the more elongated.)
The Norse moons all have retrograde orbits (they travel around Saturn in the opposite direction from the planet's rotation). That and their deviations from circular orbits and from the plane of Saturn's equator classify them as "irregular" satellites. Like Saturn's other irregular moons, they are thought to be objects that were captured by Saturn's gravity, rather than having accreted from the dusty disk that surrounded the newly formed planet as the regular satellites are thought to have done.
Unlike the Gallic and Inuit groups of Saturn's moons, the wide range of distances, inclinations and eccentricities among moons in the Norse group suggest that they are not the pieces of a single original object that shattered in a collision, but they may be the pieces of several such "original" objects.
Fenrir has a mean radius of about 2.0 km, assuming an albedo 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 22.5 million km from Saturn, the satellite takes about 1,260 Earth days to complete one orbit.
Fenrir was discovered on 12 December 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.
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 (time known as Ragnarok) and kill Odin, the supreme Norse god.