Aegir (Saturn XXXVI)

Physical characteristics

Aegir has an estimated diameter of about 6 kilometres, making it far too small to be spherical under its own gravity. Like many outer moons of the giant planets, it is likely dark and composed of a mixture of rock and ice. Detailed information on its surface composition and albedo remains limited because the object is faint and has only been observed with ground-based telescopes.

Orbit and dynamics

The moon orbits Saturn at an average distance of roughly 19.6 million kilometres and completes one revolution in about 1,026 days. Its orbit is noticeably eccentric (e ≈ 0.237) and highly inclined, with an inclination near 167° to the ecliptic (about 140° relative to Saturn's equator), confirming its retrograde motion. These orbital parameters are typical for the Norse group and point to a complex dynamical history, likely involving capture and subsequent collisional evolution.

• Semi-major axis: ~19,618,000 km
• Orbital period: ~1,025.9 days
• Inclination: ~167° to the ecliptic (retrograde)
• Eccentricity: ~0.237
• Diameter: ~6 km

Discovery and naming

Aegir was identified from observations made between December 12, 2004, and March 11, 2005; the discovery was announced on May 4, 2005. The team credited with the find included Scott S. Sheppard, David C. Jewitt, Jan Kleyna and Brian G. Marsden. For the discovery announcement and observational details see the original notices by the discoverers: discovery note 1 and discovery note 2. Additional orbital analyses were reported in follow-up circulars and databases: orbit data and orbital elements.

Name and cultural reference

In April 2007 the International Astronomical Union assigned the name Aegir, after Ægir, a figure from Norse mythology. Ægir is traditionally portrayed as a giant associated with the sea, often described as a hospitable host to the gods and a calming presence in storms. The name follows the convention of naming Saturn’s retrograde moons after Norse mythological figures; for further background on the naming decision see the relevant announcement: naming announcement. General information about the mythological Ægir and related figures can be found in broader mythological references: Norse mythology overview.

Because Aegir is small and distant, most knowledge about it comes from astrometric tracking rather than detailed imaging. Its membership in the Norse group and its orbital behavior make it of interest to researchers studying the capture, collisional fragmentation, and long-term evolution of irregular satellites around the giant planets.