Namaka (moon)

Namaka (full designation (136108) Haumea II) is the smaller, inner moon of the trans-Neptunian dwarf planet Haumea. Discovered by Michael E. Brown and the Keck Observatory adaptive optics team in the fall of 2005, it is named after Nāmaka, a water spirit and one of the daughters of Haumea in Hawaiian mythology. Namaka follows a highly elliptical orbit that is highly tilted by roughly 13 degrees with respect to Haumea's equator. Namaka is heavily perturbed by both the gravitational influence of Haumea's larger, outer moon Hiʻiaka and the variable gravitational field of Haumea's elongated shape. With a diameter of around 150 km (93 mi), Namaka is predicted to have an irregular shape and a chaotic rotation. It has a reflective surface made of fresh water ice, similar to that of Haumea and Hiʻiaka. Like Hiʻiaka, Namaka is believed to be a fragment of Haumea that was ejected in the aftermath of a giant impact 4.4 billion years ago.

Discovery and name Namaka was discovered in the fall of 2005 by Michael E. Brown and the W. M. Keck Observatory adaptive optics team at Mauna Kea, Hawaii. It was noticed in high-resolution images of Haumea taken by the Keck II telescope on 1 March, 28 May, and 30 June 2005. Namaka was not recognized earlier because it was much fainter and closer to Haumea than its brighter sibling moon Hiʻiaka, which was discovered by Brown and his team back in January 2005. The discovery of Namaka was announced by the Central Bureau of Astronomical Telegrams on 1 December 2005. When Namaka was announced, it given the temporary provisional designation S/2005 (2003 EL61) 2, which indicates it is the second moon of Haumea (then known as 2003 EL61) discovered in 2005. Brown nicknamed the moon "Blitzen," after one of Santa Claus's reindeer, as a continuation of the Christmas-themed nicknames he had been giving to the Haumea system at the time. Haumea, Hiʻiaka, and Namaka were all officially named after Hawaiian deities by the International Astronomical Union (IAU) on 17 September 2008. In Hawaiian mythology, Nāmaka is a water spirit born from the body of the fertility goddess Haumea. These names were proposed to the IAU by Brown's team in September 2006, who wanted to pay tribute to the location where they discovered the moons of Haumea.

Orbit

Namaka is the inner moon of Haumea, orbiting the dwarf planet in roughly 18.3 days at an average distance of 25,500 km (15,800 mi). Namaka follows a highly tilted elliptical orbit with an eccentricity of about 0.22 and an inclination of roughly 13° with respect to both Haumea's equator and Hiʻiaka's orbital plane. Namaka is heavily perturbed by both the gravitational influence of Hiʻiaka and the variable gravitational field of Haumea's elongated shape, which results in a time-varying eccentricity and inclination as well as nodal and apsidal precession of Namaka's orbit. The ratio of Namaka's and Hiʻiaka's orbital periods is 2.689±0.004, which means Namaka and Hiʻiaka may be in (or is close to) a 8:3 mean-motion orbital resonance with each other, where Hiʻiaka completes 3 orbits for every 8 orbits completed by Namaka. Although it is uncertain whether Namaka and Hiʻiaka are still in a 8:3 orbital resonance today, simulations have shown that this resonance has likely played a major role in producing the moons' present-day orbits by allowing Hiʻiaka to transfer its initial orbital eccentricity and inclination to Namaka over the past few hundred million years. It is uncertain how long ago this resonance began and how long it had lasted.

Effects on Haumea and its ring Like Hiʻiaka, Namaka's gravity induces a slight torque on Haumea that causes the dwarf planet's rotation axis to slowly precess by less than 1 degree. The period of Haumea's axial precession due to Namaka is equivalent to the nodal precession period of Namaka's orbit, which is several decades. Namaka's gravitational influence on Haumea's ring is small because the moon has a low mass and orbits far from Haumea's ring. Although Namaka's inclined orbit may affect the inclination of Haumea's ring and could potentially disperse the ring particles, these effects are undone by the much stronger gravitational perturbations by Haumea's flattened shape.

Observation

When viewed from Earth, Namaka is about 67 times fainter than Haumea (1.5% of Haumea's brightness) and 3.7 times fainter than Hiʻiaka (~27% of Hiʻiaka's brightness). Because Namaka orbits close to Haumea, its angular separation from Haumea is less than 1 arcsecond, so it cannot be resolved separately without the use of high-resolution telescopes like the Hubble Space Telescope. Namaka was predicted to eclipse and occult Haumea in 2009–2011, but observations did not detect any during that time. Later recalculations showed that the predicted time frame of Namaka's eclipses and occultations of Haumea were generally accurate, but are somewhat offset in time compared to previous predictions. Occasionally Namaka may pass in front of a background star and block out its light from Earth, resulting in a stellar occultation. Only one stellar occultation by Namaka has been observed on 16 March 2025.

Physical characteristics

Size, mass, and density Although the diameter of Namaka is poorly constrained, it is most likely between 100 and 200 km (62 and 124 mi), according to visible light and thermal infrared observations with inferred values for its albedo and density. Assuming an average diameter of 150 km (93 mi), Namaka would be about half the diameter of Hiʻiaka. More accurate and precise measurements of Namaka's diameter can be made if it is observed occulting a background star. Namaka has a mass of about 1×1018 kg, which is roughly ten times less massive than Hiʻiaka (~10% of Hiʻiaka's mass) and roughly 3,000 times less massive than Haumea (~0.03% of Haumea's mass). The mass of Namaka was determined by observing deviations in the orbits of Namaka and Hiʻiaka due to their gravitational influence on each other. If Namaka has a diameter of 150 km (93 mi), its mass would indicate a low bulk density of 0.650 g/cm3, which could suggest that Namaka has a highly porous interior similar to other small trans-Neptunian objects.

Surface Like Haumea and Hiʻiaka, Namaka's surface is mostly made of fresh water ice. While the geometric albedo or reflectivity of Namaka's surface has not been measured, it is most likely between 50% and 80%.

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