Namaka (moon)

Namaka was discovered in the fall of 2005^[a] by Michael E. Brown and the W. M. Keck Observatory adaptive optics team at Mauna Kea, Hawaii.^[2][11] It was noticed in high-resolution images of Haumea taken by the Keck II telescope on 1 March, 28 May, and 30 June 2005.^{[2][12]: L44 } 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.^[11] The discovery of Namaka was announced by the Central Bureau of Astronomical Telegrams on 1 December 2005.^[2]

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.^[2] 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.^[13][11][d]

Haumea, Hiʻiaka, and Namaka were all officially named after Hawaiian deities by the International Astronomical Union (IAU) on 17 September 2008.^[14] In Hawaiian mythology, Nāmaka is a water spirit born from the body of the fertility goddess Haumea.^[14] 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.^[15]

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).^{[5]: 6 [6]: 4770 } 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 HiÊ»iaka's orbital plane.^{[5]: 6 } Namaka is heavily perturbed by both the gravitational influence of HiÊ»iaka and the variable gravitational field of Haumea's elongated shape, which results in a time-varying eccentricity and inclination^{[6]: 4773 } as well as nodal and apsidal precession of Namaka's orbit.^{[5]: 9 }

The ratio of Namaka's and Hiʻiaka's orbital periods is 2.689±0.004,^{[5]: 15 } 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.^{[6]: 4770 } Although it is uncertain whether Namaka and Hiʻiaka are still in a 8:3 orbital resonance today,^{[5]: 15 } 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.^{[16]: 8 } It is uncertain how long ago this resonance began and how long it had lasted.^{[16]: 8, 13 }

When viewed from Earth, Namaka is about 67 times fainter than Haumea (1.5% of Haumea's brightness^{[7]: 44 [e]}) and 3.7 times fainter than Hiʻiaka (~27% of Hiʻiaka's brightness^[f]).^{[7]: 44 [6]: 4769 } Because Namaka orbits close to Haumea, its angular separation from Haumea is less than 1 arcsecond,^{[6]: 4771 } so it cannot be resolved separately without the use of high-resolution telescopes like the Hubble Space Telescope.^{[18]: L1 }

Namaka was predicted to eclipse and occult Haumea in 2009–2011,^[6] but observations did not detect any during that time.^{[19]: 4 } 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.^{[5]: 14 } 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.^[8]

Namaka and Hiʻiaka are widely believed to be fragments of Haumea that were ejected in the aftermath of a giant impact 4.4 billion years ago (77–82 million years after the formation of the Solar System), when Neptune was migrating outward and gravitationally scattering objects in the Kuiper belt.^{[23]: 1–2, 14 } This impact event is hypothesized to involve two large Kuiper belt objects of similar size, which obliquely collided with each other and merged into a single, rapidly rotating body that eventually deformed into an ellipsoidal body, becoming Haumea today.^{[23]: 2 } While this hypothesis explains Haumea's rapid rotation and high bulk density, it fails to explain the existence of Haumea's moons and family of icy KBOs on similar orbits, because such an energetic impact would have ejected fragments at speeds several times Haumea's escape velocity.^{[23]: 2 }

Rather than having formed directly from a giant impact, Haumea's family and moons are instead believed to have been ejected via rotational fissioning of Haumea roughly 80 million years after the impact (147–162 million years after Solar System's formation).^{[5]: 15 [23]: 1, 14 } A 2022 study led by Jessica Noviello and collaborators proposed that Haumea continued differentiating and growing its rocky core after the giant impact, which led to a gradual speed-up of Haumea's rotation rate as a consequence of angular momentum conservation.^[23] Centrifugal forces on Haumea's equator eventually grew so great that icy surface material began ejecting into orbit around Haumea, forming a disk of material that eventually coalesced into moons.^{[23]: 2–3 } About 3% of Haumea's initial mass and 14% of its initial angular momentum were lost via rotational fissioning.^{[23]: 1 }

The accretion of material around Haumea was likely gentle, as hinted by Namaka's present-day low density.^{[5]: 10 } Some of the moons that formed from this disk gravitationally scattered each other and escaped the Haumea system to become Haumea family KBOs, while the remaining material in orbit around Haumea became Namaka, Hiʻiaka, and Haumea's ring.^{[23]: 2–3 [5]: 15 } Namaka and Hiʻiaka likely formed with initially coplanar and circular orbits near their present-day locations from Haumea.^{[16]: 8, 13 [5]: 15 } Subsequent orbital evolution by tidal interactions with Haumea and perturbations by closely-passing trans-Neptunian objects eventually led to Namaka and Hiʻiaka entering a 8:3 mean-motion orbital resonance, which led to their present-day orbits.^{[16]: 8, 13 [5]: 15 }

• HiÊ»iaka – the larger moon of Haumea
• Haumea family – a population of water-ice rich Kuiper belt objects that were ejected from Haumea 4.4 billion years ago