Hiʻiaka is above Haumea (center) in this Keck telescope image.
|Discovered by||Michael E. Brown, |
David Rabinowitz, et al.
|Discovery date||26 January 2005|
|MPC designation||Haumea I Hiʻiaka|
|(136108) 2003 EL61 I, |
S/2005 (2003 EL61) 1
|Mass||(1.79±0.11)×1019 kg (0.45% of Haumea)|
|20.3 (3.0 difference from primary's 17.3)|
Hiʻiaka is the larger, outer moon of the possible dwarf planet Haumea. It is named after one of the daughters of Haumea, Hiʻiaka, the patron goddess of the Big Island of Hawaii. It orbits once every 49.12±0.03 d at a distance of 49880±198 km, with an eccentricity of 0.0513±0.0078 and an inclination of 126.356±0.064°. 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, very possibly Varda I Ilmarë, and perhaps Salacia I Actaea.
Its measured brightness is 5.9±0.5%, 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 Hi'iaka 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 Hi'iaka is not a gravitationally collapsed spheroid; they further suggest that Hi'iaka is not tidally locked and has a rotation period of about 9.8 hours.
The near infrared spectrum of Hiʻiaka 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.