Aegir 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.


Aegir has a mean radius of 1.9 miles (3 km), assuming an albedo (a measure of how reflective the surface is) of 0.04. It orbits Saturn at an inclination of about 167 degrees and an eccentricity of 0.25. At a mean distance of 12.9 million miles (20.7 million km) from Saturn, the moon takes about 1,118 Earth days to complete one orbit. Aegir 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. Aegir and the other Norse moons also have eccentric orbits, meaning they are more elongated than circular.

Like Saturn's other irregular moons, Aegir 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 Aegir Got its Name

Originally called S/2004 S10, Aegir was named for a giant in Norse mythology who personified the ocean.

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