Bergelmir, formally designated Saturn XXXVIII and provisionally S/2004 S 15, is one of Saturn’s many small outer moons. It is classified among the irregular satellites because of its distant, inclined, and retrograde orbit. The object was reported in 2005 by a team of astronomers that included Scott S. Sheppard and David C. Jewitt; the announcement credited observations taken over several months that revealed its faint motion against the background stars. See the discovery announcement by Scott S. Sheppard and colleagues and the related report by David C. Jewitt.

Physical characteristics and orbit

Bergelmir is very small—roughly 6 kilometres across—so it is almost certainly non-spherical and too tiny to retain any appreciable atmosphere. It orbits Saturn at an average distance of about 19,372,000 kilometres and completes a revolution in approximately 1006.659 days. Its orbit is both inclined and eccentric: the inclination is about 157° to the ecliptic (about 134° relative to Saturn’s equator) and the orbital eccentricity is near 0.152, values that place it among Saturn’s retrograde irregular satellites. The inclination and eccentricity measurements are reported in the discovery material and subsequent orbit determinations (inclination reference, eccentricity reference).

As with other small outer moons, Bergelmir is expected to have a dark, heavily cratered surface and a low albedo, although detailed photometric or spectral data are limited because the moon is faint and distant. Its size and orbit make it unlikely to be native to Saturn’s original accretion disk; instead, it is generally thought to be a captured object or a fragment produced in collisional events among captured bodies.

Discovery and observational history

The object was detected from images taken between 12 December 2004 and 9 March 2005 and announced on 4 May 2005 by the discovery team of Sheppard, Jewitt, Jan Kleyna and Brian G. Marsden. Their work involved repeated imaging to confirm that the moving point of light shared a bound orbit about Saturn rather than being a transient asteroid or background source. Continued follow-up observations refined Bergelmir’s orbital elements and confirmed its long-term stability as a satellite of Saturn.

Name and mythological origin

In April 2007 the moon received the name Bergelmir, drawn from Norse mythology. Bergelmir is described in the mythic sources as a giant and the grandson of Ymir; according to the tradition he and his wife were the sole survivors of a great flood of blood that followed Ymir’s slaying. Bergelmir’s survival allowed a new line of giants to arise. The naming follows the convention of giving retrograde irregular satellites of Saturn names from Norse myth: see the entry for Bergelmir as well as related figures such as Ymir and the role of Odin in the cosmogony.

Importance and context

  • Designation: Saturn XXXVIII (S/2004 S 15).
  • Diameter: ~6 km (estimated).
  • Semi-major axis: ~19,372,000 km; period: ~1006.7 days.
  • Inclination: ~157° to the ecliptic; eccentricity: ~0.152.
  • Group: Norse (retrograde irregular satellites).
  • Discovery: announced May 4, 2005 by Sheppard, Jewitt, Kleyna and Marsden.

Although Bergelmir is one of the smaller and more distant members of Saturn’s entourage, studying such bodies helps astronomers understand the population of captured objects around the giant planets, the collisional history of the outer satellite system, and the processes that shape small, irregular moons. Because Bergelmir is faint, future improvements in orbit determination and physical characterization depend on targeted observations with large telescopes and careful astrometry.