Hiʻiaka (moon)
Larger outer moon of dwarf planet Haumea, composed of crystalline water ice.
Hiʻiaka, also known as (136108) Haumea I, is the larger and more distant of the two moons orbiting the dwarf planet Haumea, which lies beyond Neptune. It was spotted on January 26, 2005, by Michael E. Brown and the adaptive optics team at the Keck Observatory. Its name comes from Hiʻiaka, the Hawaiian goddess who protects the Big Island and is a daughter of Haumea. The moon travels around Haumea in a slightly oval-shaped path, completing one orbit every 49.5 days at an average distance of 49,400 km.
This moon is elongated and irregular, with an average diameter of about 370 km, making it the sixth-largest known moon of any trans-Neptunian object. Its density is very low at 0.64 g/cm³, suggesting it is mostly made of loosely packed water ice and rock. Telescope observations reveal a highly reflective surface of crystalline water ice, much like Haumea itself. Hiʻiaka spins once on its axis every 9.68 hours. Along with its smaller sibling moon Namaka, it is thought to be a chunk of Haumea that was blasted away during a giant collision about 4.4 billion years ago.
**Discovery**
Hiʻiaka was the first moon found around Haumea, discovered on January 26, 2005, by Michael E. Brown and the Keck Observatory adaptive optics team on Mauna Kea, Hawaii. Since Haumea’s discovery hadn’t been announced yet, Hiʻiaka’s discovery was made public later, on July 29, 2005. At that time, it received the temporary label S/2005 (2003 EL61) 1, meaning it was the first moon of Haumea (then called 2003 EL61) found in 2005. Brown had been calling Haumea “Santa,” so he nicknamed Hiʻiaka “Rudolph,” after one of Santa’s reindeer.
Haumea, Hiʻiaka, and Namaka were officially given Hawaiian deity names by the International Astronomical Union on September 17, 2008. In Hawaiian myth, Hiʻiaka is the goddess of hula and the daughter of the fertility goddess Haumea. Brown’s team proposed these names in September 2006 to honor the location where the moons were discovered.
**Physical characteristics**
**Size, mass, and density**
Stellar occultations by Hiʻiaka on April 6 and 16, 2021, showed it is an elongated, ellipsoid-shaped object measuring about 480 km by 360 km by 286 km. This gives it a volume-equivalent diameter of 370 km. It ranks as the sixth-largest known moon of a trans-Neptunian object, after Charon (1,212 km), Dysnomia (615 km), Vanth (443 km), Ilmarë (403 km), and Actaea (393 km)
- Discovered
- 26 January 2005
- Discoverer
- Michael E. Brown and the Keck Observatory adaptive optics team
- Mean diameter
- 370 km (230 mi)
- Orbital period
- 49.5 days
- Rotation period
- 9.68 hours
- Bulk density
- 0.64 g/cm³
- Geometric albedo
- 0.74
Lore & Background
Hiʻiaka was the first satellite discovered around Haumea, though the discovery of Haumea itself was not made public until later. The moon was given the provisional designation S/2005 (2003 EL61) 1, and Brown nicknamed it 'Rudolph' after one of Santa Claus's reindeer. On 17 September 2008, the International Astronomical Union officially named it Hiʻiaka after the Hawaiian goddess of hula, daughter of the fertility goddess Haumea.
Hiʻiaka is an elongated, irregularly shaped body with dimensions of 480 km × 360 km × 286 km, as revealed by stellar occultations in April 2021. Its very low density of 0.64 g/cm³ indicates a highly porous interior composed mostly of loosely-packed water ice and rock. The moon is not in hydrostatic equilibrium, likely due to high material strength, and its rotation is not tidally locked to Haumea.
Spectroscopic observations show Hiʻiaka's surface is dominated by crystalline water ice, with deep absorption features at 1.5 μm and 2.0 μm. Its geometric albedo of 0.74 is higher than Haumea's, possibly due to larger water ice grains on its surface. The moon is believed to have formed from icy fragments ejected during rotational fissioning of Haumea about 4.4 billion years ago.
Reader's Guide
Hiʻiaka is significant as one of the largest known moons of a trans-Neptunian object and as a key piece of evidence for the collisional origin of the Haumea system. Its composition, nearly identical to Haumea's, supports the hypothesis that both moons and the Haumea family of Kuiper belt objects originated from a giant impact followed by rotational fission. The moon's high albedo and crystalline water ice surface raise questions about resurfacing mechanisms in the outer Solar System. Hiʻiaka's elongated shape and lack of hydrostatic equilibrium provide constraints on the internal structure and material strength of small icy bodies. Its orbit and rotation, measured through light curves and occultations, help refine models of tidal evolution and spin dynamics in the Kuiper belt. The moon's discovery and naming also highlight the role of Hawaiian mythology in astronomical nomenclature.
Did You Know?
- Hiʻiaka was nicknamed 'Rudolph' by discoverer Michael Brown before its official naming.
- The moon's rotation period of 9.68 hours is not tidally locked to Haumea.
- Hiʻiaka's surface is made of crystalline water ice, with a geometric albedo of 0.74.
- It is believed to have formed from icy fragments ejected during Haumea's rotational fission about 4.4 billion years ago.
More in Discoveries by Michael E. Brown 1-24
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