Hiiaka is the larger, outer moon of the trans-Neptunian dwarf planet Haumea. It is named after one of the daughters of Haumea, Hiiaka, the patron goddess of the Big Island of Hawaii. It orbits once every 49.12 days at a distance of 49,880 km, with an eccentricity of 0.051 and an inclination of °126.356. Assuming its estimated diameter of over 300 km is accurate, it may be the fourth- or fifth-largest known moon of a Trans-Neptunian object, after Pluto I Charon, Eris I Dysnomia, Orcus I Vanth, and possibly Varda I Ilmarë and Salacia I Actaea.

Discovery

Hiiaka was the first satellite discovered around Haumea. It was discovered on 26 January 2005 and nicknamed "Rudolph" by the discovery team before being assigned an official name.

Physical characteristics

Size and brightness

Its measured brightness is %5.9, translating into a diameter of about 22% of its primary, or in the range of 320 km, assuming similar infrared albedo. To put this in perspective, if Hiʻiaka were in the asteroid belt, it would be larger than all but the four largest asteroids, after 1 Ceres, 2 Pallas, 4 Vesta, and 10 Hygiea. In spite of its relatively large size, however, lightcurve studies suggest that Hiʻiaka is not a gravitationally collapsed spheroid; they further suggest that Hiʻiaka is not tidally locked and has a rotation period of about 9.8 hours.

Mass

The mass of Hiiaka is estimated to be 1.79 kg using precise relative astrometry from the Hubble Space Telescope and Keck Telescope and applying 3-body, point-mass model to the Haumean system.

Spectrum and composition

The near infrared spectrum of Hiiaka is dominated by water-ice absorption bands, which means that its surface is made mainly of water ice. The presence of the band centered at 1.65 μm indicates that the surface water ice is primarily in the crystalline form. Currently it is unclear why water ice on the surface has not turned into amorphous form as would be expected due to its constant irradiation by cosmic rays.