Greip (provisional designation S/2006 S 4, also Saturn LI) is a small irregular satellite of Saturn discovered in 2006. It orbits far from the planet in a retrograde, highly inclined and eccentric path typical of the outer irregular moons. Because of its size and remote orbit it is faint and has been studied mainly by telescopic surveys rather than by spacecraft flybys.

Discovery and name

The existence of Greip was announced on June 26, 2006 following observations taken between January and May of that year. The discovery team included Scott S. Sheppard and David C. Jewitt, together with collaborators Jan Kleyna and Brian G. Marsden. After confirmation of its orbit the moon received the permanent Roman numeral designation Saturn LI and was later named after the giantess Greip from Norse mythology, consistent with the naming convention for Saturn's retrograde irregular satellites.

Orbit and classification

Greip is classified among Saturn's irregular, retrograde moons often grouped with the Norse satellites. It orbits at an average distance of roughly 18,066,000 km from Saturn and completes one revolution in about 906.6 days. Its orbital plane is steeply tilted: an inclination of about 172.7° to the ecliptic (approximately 159.2° to Saturn's equator) places it on a retrograde track (inclination data). The orbit is noticeably non-circular, with an eccentricity near 0.37 (eccentricity), which causes significant variation in its distance from Saturn over a single orbit.

Physical characteristics

Greip is very small by planetary satellite standards, with an estimated diameter of about 6 kilometres. Like other tiny outer satellites its shape is probably irregular rather than spherical, and its surface is expected to be dark and heavily cratered, showing little or no geologic activity. Because of its faintness only limited photometric and color information is available from ground-based telescopes.

Origin and scientific importance

Greip and similar irregular moons are generally believed to be captured objects, either fragments of a larger body broken up by collisions or small bodies that were gravitationally trapped during the early solar system. Studying their orbits, sizes, and surface properties helps astronomers reconstruct the collisional history and dynamical evolution of Saturn's satellite system and the outer solar system in general.

Observation and future study

Observations to date have relied on deep ground-based imaging and careful astrometric tracking to refine Greip's orbit. Its small size and distant orbit make direct spacecraft study unlikely in the near term, but improvements in telescopic sensitivity and continued monitoring can refine its physical and dynamical properties. Cataloguing such moons and their relationships remains important for understanding capture mechanisms and the population of irregular satellites around the giant planets.

  • Designation: Saturn LI (S/2006 S 4)
  • Discovery: announced June 26, 2006 by Scott S. Sheppard, David C. Jewitt, Jan Kleyna and Brian G. Marsden
  • Mean distance: ~18,066,000 km from Saturn
  • Orbital period: ~906.6 days
  • Inclination: ~172.7° to the ecliptic (details)
  • Eccentricity: ~0.3735 (details)
  • Diameter: ~6 km