Norse group

The Norse group (or family or cluster; also simply referred to as the retrograde moons) comprises the retrograde irregular satellites of Saturn. They are unlikely to have a common origin as a single collisional family and their orbital parameters are very widely dispersed; more probably they are composed of a number of dynamical clusters with more homogeneous orbital and physical parameters. As of April 2026, there are 210 known members, making it by far the largest group of Saturn's confirmed moons. The Norse group is dominated by moons that are smaller in size, which could be indicative of a recent catastrophic collision event within the population.

Naming The International Astronomical Union (IAU) reserves names taken from Norse mythology (mostly giants) for the retrograde satellites of Saturn, hence the name Norse group. The term Norse group is only sometimes used as authors do not consider them to be dynamically related like a collisional family, and they may refer to them simply as the retrograde satellites. The exception to the moon naming scheme is Phoebe, whose name is taken from Greek mythology, which was discovered and named long before the others. Only 31 of the moons have names at present. The discovery of 17 new satellites in this group was announced in October 2019. A team led by Scott S. Sheppard using the Subaru Telescope at Mauna Kea discovered 20 new moons, each about 5 kilometres (3 miles) in diameter. 17 of these fit into the Norse group, one of which was the most distantly orbiting satellite of Saturn known. A public naming contest for the satellites was announced, restricted to names from Norse mythology. Ten of the satellites received official names in August 2022.

General characteristics

Overall, their orbital elements are very broadly distributed, with inclinations between 136° and 178°. The retrograde irregular moons orbit at larger distances from Saturn on average than the prograde moons, which is a common trend for irregular satellite systems in general. The retrograde satellites were found to have generally faster rotation periods than the progrades; every prograde irregular measured had a rotation period longer than 10 hours, while most retrogrades had periods shorter than that. Dependencies between the rotation rates and the inclinations and semi-major axes were found as well. The Saturnian irregular moon population has a steep size distribution, meaning it has a large proportion of small moons relative to larger ones. It has been suggested there was a recent catastrophic collision event that could explain the large number of small moons. The cause of the steep size distribution was later narrowed down solely to a specific section of the retrograde population, proposed as the "Mundilfari subgroup". The role of Phoebe, as the largest irregular moon of Saturn, is significant. While it is unlikely for the entirety of the retrograde moons to be the collisional family of Phoebe, it is accepted that it should be involved in the plurality of impacts among the irregulars due to its large diameter. There are different hypotheses on what it implies for a potential collisional family. One argument is that members of the group would be later removed by impact with Phoebe due to sharing similar orbital elements, and therefore a surviving family is unlikely, while another viewpoint is that the high number of collision events involving it means some collisional family likely exists.

Subcategorization Several attempts have been made over an extended period to divide the retrograde satellites of Saturn into subcategories based on the satellites' orbital characteristics. In their 2001 article reporting on the discovery of the first irregular moons around Saturn since that of Phoebe, Gladman et al. separated by inclination four of the five newly discovered retrograde satellites (later named Ymir, Thrymr, Mundilfari, and Suttungr) in a group with Phoebe called the "Phoebe group". They tentatively placed the last moon (Skathi) on its own due to its differing inclination, though Mundilfari's inclination also differed enough from the rest of the group to be questionable. A later study disputed Ymir's membership, determining that its colour and semi-major axis made it incompatible with having originated from Phoebe in a collision. They did not attempt to validate the membership of the other three moons. In 2008, Nicholson et al. split the retrograde satellites into three groups, but made no attempt to justify their potential shared dynamic origins, simply roughly sorting them by inclination alone. Each group was centred on a moon and their inclination. Their groups were the Phoebe group with an inclination around 175° (Phoebe, Suttungr, Thrymr, Ymir, and Fornjot), the Mundilfari group with an inclination at 168° (Mundilfari, Saturn LXVII, Aegir, S/2004 S 12, S/2004 S 13, Hati, Fenrir, and S/2004 S 17), and the Skathi group with an inclination at 153° (Skathi, Narvi, Farbauti, Bergelmir, and Bestla). They did not provide exhaustive lists of all members in their groups, and commented that nine newly discovered satellites at the time all fell into their inclination groupings but did not specify which moons went into which groups. Also in 2008, Turrini et al. pointed out that members from a potential Phoebe family would likely be collisionally removed by the moon itself, which argues against the existence of the family. As half of all potential collisions between irregular moons would involve Phoebe due to its large size, this produces a "sweeping effect" that they presumed was probably the cause of a lack of known moons with semi-major axes around Phoebe's. They proposed that it was not related to any of the other known irregular moons, noting that none of the other satellites had compatible colours. Using an algorithm called the Hierarchical Clustering Method, they found several retrograde "families", which did not cover all 27 of the known retrograde satellites at the time. A group was qualified as a viable family if it passed the condition that the members were close enough in their orbital elements to potentially have a shared collisional origin with realistic dispersion velocities (≤200 m/s). Six of these were found to be acceptably realistic. The six groups were:

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