120347 Salacia

Lore & Background

Salacia was discovered on 22 September 2004 at the Palomar Observatory, with its discovery formally announced in May 2005. Precovery images date back to 25 July 1982 from the Siding Spring Observatory. It orbits the Sun at an average distance slightly greater than Pluto, with a semi-major axis of 42.2 AU and an orbital period of 274.0 years. Its orbit has a moderate eccentricity of 0.11 and a high inclination of 23.9°, placing it in the hot classical Kuiper belt population, though some frameworks classify it as a scattered–extended object. Salacia has a low light curve amplitude of about 3%, implying a nearly spherical shape. Its surface is dark gray with a neutral color index and an exceptionally low albedo of 0.041±0.004, indicating a lack of geological activity. JWST observations show water ice makes up less than 5% of its surface, with both amorphous and crystalline water ice and carbon dioxide ice detected, but no volatile super-ices like methane. It has one known moon, Actaea, and the two bodies form a fully tidally locked system with a synchronous rotation period of 5.49 days. A rare mutual occultation season between Salacia and Actaea will begin in 2067.

Reader's Guide

Salacia is significant as a possible dwarf planet in the Kuiper belt, offering insights into the composition and dynamics of trans-Neptunian objects. Its low albedo and dark gray surface suggest a lack of geological activity, while its density measurements—ranging from 1.45 g/cm3 to 1.50±0.12 g/cm3—indicate a non-porous interior. The presence of water ice and carbon dioxide ice, but absence of methane, places it in the 'prominent water' TNO group. Its fully tidally locked system with Actaea, similar to Pluto–Charon, provides a rare example of binary dynamics among Kuiper belt objects. Stellar occultations, including a rare double occultation in 2024, have refined its size and orbit, with future events like the 2027 occultation targeted by the RECON network. The ongoing debate over its classification—hot cubewano versus scattered–extended—highlights the complexity of categorizing outer Solar System bodies.

Did You Know?

• Salacia was discovered on 22 September 2004 using the Hale Reflector at Palomar Observatory.
• Its moon Actaea is named after one of the 50 Nereid sea-nymphs, matching the system's marine naming theme.
• A rare double stellar occultation by Salacia and Actaea was recorded on 17 August 2024.