Namaka (moon)
Namaka 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 Hiiaka and the variable gravitational field of Haumea's elongated shape.
With a diameter of around, 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 Hiiaka. Like Hiiaka, 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 Hiiaka, 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, which indicates it is the second moon of Haumea 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, Hiiaka, and Namaka were all officially named after Hawaiian deities by the International Astronomical Union 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. Namaka follows a highly tilted elliptical orbit with an eccentricity of about 0.22 and an inclination of roughly with respect to both Haumea's equator and Hiiaka's orbital plane. Namaka is heavily perturbed by both the gravitational influence of Hiiaka 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 Hiiaka's orbital periods is, which means Namaka and Hiiaka may be in a 8:3 mean-motion orbital resonance with each other, where Hiiaka completes 3 orbits for every 8 orbits completed by Namaka. Although it is uncertain whether Namaka and Hiiaka 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 Hiiaka 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 Hiiaka, 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 and 3.7 times fainter than Hiiaka. 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, according to visible light and thermal infrared observations with inferred values for its albedo and density. Assuming an average diameter of, Namaka would be about half the diameter of Hiiaka. 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, which is roughly ten times less massive than Hiiaka and roughly 3,000 times less massive than Haumea. The mass of Namaka was determined by observing deviations in the orbits of Namaka and Hiiaka due to their gravitational influence on each other. If Namaka has a diameter of, its mass would indicate a low bulk density of, which could suggest that Namaka has a highly porous interior similar to other small trans-Neptunian objects.
Surface
Like Haumea and Hiiaka, 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%.Rotation and shape
Namaka is expected to have an irregular shape because of its small size. Observations by the Hubble Space Telescope in 2008 suggest that Namaka has an elongated shape because its brightness fluctuates by 0.3 magnitudes. However, Hubble observations from 2009 and 2010 did not detect any significant periodic fluctuation in Namaka's brightness, which could suggest that Namaka either has a highly tilted rotation axis, or a slow rotation with a period greater than 1 day.Because Namaka orbits close to Haumea, the dwarf planet's tidal forces are expected to have slowed down Namaka's rotation. However, if Namaka rotates slowly, then Namaka would have a chaotic rotation because its eccentric orbit causes it to experience multiple spin-orbit resonances induced by Haumea's tides. Simulations predict that Namaka's axial tilt and rotation period can vary unpredictably over timescales of years, much like Saturn's chaotically rotating moon Hyperion. Confirming the Namaka's slow and chaotic rotation will be difficult as it requires a long time span of high-resolution observations.
Origin
Namaka and Hiiaka are widely believed to be fragments of Haumea that were ejected in the aftermath of a giant impact 4.4 billion years ago, when Neptune was migrating outward and gravitationally scattering objects in the Kuiper belt. 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. 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.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. 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. 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. About 3% of Haumea's initial mass and 14% of its initial angular momentum were lost via rotational fissioning.
The accretion of material around Haumea was likely gentle, as hinted by Namaka's present-day low density. 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, Hiiaka, and Haumea's ring. Namaka and Hiiaka likely formed with initially coplanar and circular orbits near their present-day locations from Haumea. Subsequent orbital evolution by tidal interactions with Haumea and perturbations by closely-passing trans-Neptunian objects eventually led to Namaka and Hiiaka entering a 8:3 mean-motion orbital resonance, which led to their present-day orbits.