Bestla (// BEST-lə) or Saturn XXXIX (provisional designation S/2004 S 18) is a retrograde irregular moon of Saturn. Its discovery was announced by Scott S. Sheppard, David C. Jewitt, Jan Kleyna, and Brian G. Marsden on 4 May 2005, from observations taken between 13 December 2004 and 5 March 2005.
Bestla is about 7 kilometres in diameter, and orbits Saturn at an average distance of 20,192,000 km in 1088 days, at an inclination of 147° to the ecliptic (151° to Saturn's equator), in a retrograde direction and with an eccentricity of 0.5145. Early observations from 2005 suggested that Bestla had a very high eccentricity of 0.77. Like many of the outer irregular moons of the giant planets, Bestla's eccentricity may vary as a result of the Kozai mechanism.
- Saturn's Known Satellites (by Scott S. Sheppard)
- Jewitt's New Satellites of Saturn page
- IAUC 8523: New Satellites of Saturn May 4, 2005 (discovery)
- IAUC 8826: Satellites of Jupiter and Saturn April 5, 2007 (naming the moon)