Aegir 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.
Aegir has a mean radius of about 3.0 km, assuming an albedo of 0.04. It orbits Saturn at an inclination of about 167degrees and an eccentricity of about 0.3. At a mean distance of 20.7 million km from Saturn, the satellite takes about 1,118 Earth days to complete one orbit.
Aegir 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 Aegir Got its Name:
Originally called S/2004 S10, Aegir was named for a giant in Norse mythology who personified the ocean.