 In this photo taken by theHubble Space Telescope, Namaka is the faint spot near the bottom, directly belowHaumea (center). | |
| Discovery[1][2] | |
|---|---|
| Discovered by | |
| Discovery site | W. M. Keck Obs.,Mauna Kea |
| Discovery date | 2005[a] |
| Designations | |
Designation | (136108) Haumea II[3][4] |
| Pronunciation | /nɑːˈmɑːkə/ Hawaiian:[naːˈmɐkə] |
Named after | Nāmaka |
| S/2005 (2003 EL61) 2[2] S/2005 (136108) 2[3] | |
| Orbital characteristics[b] | |
| Epoch 28 May 2008 12:00UT (JD 2454615.0) | |
| 25506±36 km | |
| Eccentricity | 0.2179+0.0033 −0.0032 |
| 18.2783±0.0076 d[6]: 4770 | |
| 185.19°+0.69° −0.65° | |
| Inclination |
|
| 23.725°+0.149° −0.150° (toecliptic) | |
| 118.35°+0.39° −0.42° | |
| Satellite of | Haumea |
| Physical characteristics | |
| 150±50 km[7]: 44 ≥ 83±2 km (occultation)[8] | |
| Mass | (1.18±0.25)×1018 kg[5]: 6 |
Meandensity | ~0.65 g/cm3 (for150 km diameter)[5]: 10 |
| unknown, likelychaotic[5]: 11–14 | |
| unknown, likelychaotic[5]: 11–14 | |
| Albedo | 0.5 to 0.8[7]: 45 |
| Temperature | ≈40 K (same as Haumea)[9]: 8 |
| ~22[c] | |
| 5.1[7]: 44 | |
Namaka (full designation(136108) Haumea II) is the smaller, inner moon of thetrans-Neptuniandwarf planetHaumea. Discovered byMichael E. Brown and theKeck Observatoryadaptive optics team in the fall of 2005, it is named afterNāmaka, awater spirit and one of the daughters ofHaumea inHawaiian mythology. Namaka follows a highlyelliptical orbit that is highlytilted by roughly 13degrees with respect to Haumea's equator. Namaka is heavilyperturbed by both the gravitational influence of Haumea's larger, outer moonHiʻiaka and the variablegravitational field of Haumea's elongated shape.[5]
With a diameter of around 150 km (93 mi), Namaka is predicted to have an irregular shape and achaotic rotation. It has a reflective surface made of freshwater 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.
Namaka was discovered in thefall of 2005[a] byMichael E. Brown and theW. M. Keck Observatoryadaptive optics team atMauna 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 moonHiʻiaka, which was discovered by Brown and his team back in January 2005.[11] The discovery of Namaka was announced by theCentral Bureau of Astronomical Telegrams on 1 December 2005.[2]
When Namaka was announced, it given the temporaryprovisional designationS/2005 (2003 EL61) 2, which indicates it is the second moon of Haumea (then known as2003 EL61) discovered in 2005.[2] Brown nicknamed the moon "Blitzen," after one ofSanta Claus's reindeer, as a continuation of theChristmas-themed nicknames he had been giving to the Haumea system at the time.[13][11][d]
Haumea, Hiʻiaka, and Namaka were all officially named afterHawaiian deities by theInternational Astronomical Union (IAU) on 17 September 2008.[14] InHawaiian mythology,Nāmaka is awater spirit born from the body of thefertility goddessHaumea.[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 tiltedelliptical orbit with aneccentricity of about 0.22 and aninclination of roughly13° with respect to both Haumea's equator and Hiʻiaka's orbital plane.[5]: 6 Namaka is heavilyperturbed by both the gravitational influence of Hiʻiaka and the variablegravitational field of Haumea's elongated shape, which results in a time-varying eccentricity and inclination[6]: 4773 as well asnodal andapsidal precession of Namaka's orbit.[5]: 9
The ratio of Namaka's and Hiʻiaka's orbital periods is2.689±0.004,[5]: 15 which means Namaka and Hiʻiaka may be in (or is close to) a 8:3mean-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
Like Hiʻiaka, Namaka's gravity induces a slighttorque on Haumea that causes the dwarf planet'srotation axis to slowlyprecess by less than 1 degree.[5]: 11 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.[5]: 11–12 Namaka's gravitational influence on Haumea's ring is small because the moon has a low mass and orbits far from Haumea's ring.[17][5]: 12 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.[17]: 4–5
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, itsangular separation from Haumea is less than 1arcsecond,[6]: 4771 so it cannot be resolved separately without the use of high-resolution telescopes like theHubble Space Telescope.[18]: L1
Namaka was predicted toeclipse andoccult 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 astellar occultation. Only one stellar occultation by Namaka has been observed on 16 March 2025.[8]
Although the diameter of Namaka is poorly constrained,[5]: 10 it is most likely between 100 and 200 km (62 and 124 mi), according to visible light andthermal infrared observations with inferred values for itsalbedo and density.[7]: 44 Assuming an average diameter of 150 km (93 mi),[5]: 10 Namaka would be about half the diameter of Hiʻiaka.[7]: 44 More accurate and precise measurements of Namaka's diameter can be made if it is observed occulting a background star.[8][5]: 10
Namaka has a mass of about1×1018 kg, which is roughly ten times less massive than Hiʻiaka[17]: 4 (~10% of Hiʻiaka's mass) and roughly 3,000 times less massive than Haumea (~0.03% of Haumea's mass).[5]: 6 The mass of Namaka was determined by observing deviations in the orbits of Namaka and Hiʻiaka due to their gravitational influence on each other.[5]: 5 If Namaka has a diameter of 150 km (93 mi), its mass would indicate a lowbulk density of0.650 g/cm3, which could suggest that Namaka has a highlyporous interior similar to other smalltrans-Neptunian objects.[5]: 10
Like Haumea and Hiʻiaka, Namaka's surface is mostly made of freshwater ice.[20] While thegeometric albedo or reflectivity of Namaka's surface has not been measured, it is most likely between 50% and 80%.[7]: 45
Namaka is expected to have an irregular shape because of its small size.[19]: 4 Observations by theHubble Space Telescope in 2008 suggest that Namaka has an elongated shape because its brightness fluctuates by 0.3magnitudes.[18]: L2–L3 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 highlytilted rotation axis, or a slow rotation with aperiod greater than 1 day.[19]: 4
Because Namaka orbits close to Haumea, the dwarf planet'stidal forces are expected to have slowed down Namaka's rotation.[5]: 11 However, if Namaka rotates slowly, then Namaka would have achaotic rotation because its eccentric orbit causes it to experience multiplespin-orbit resonances induced by Haumea's tides.[19]: 4 [5]: 11 Simulations predict that Namaka's axial tilt and rotation period can vary unpredictably over timescales of years, much likeSaturn's chaotically rotating moonHyperion.[5]: 11, 14 Confirming the Namaka's slow and chaotic rotation will be difficult as it requires a long time span of high-resolution observations.[5]: 11
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 theformation of the Solar System), whenNeptune wasmigrating outward and gravitationally scattering objects in theKuiper belt.[23]: 1–2, 14 Thisimpact 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 anellipsoidal 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 andfamily of icy KBOs on similar orbits, because such an energetic impact would have ejected fragments at speeds several times Haumea'sescape 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 continueddifferentiating and growing its rocky core after the giant impact, which led to a gradual speed-up of Haumea's rotation rate as a consequence ofangular 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 initialangular momentum were lost via rotational fissioning.[23]: 1
Theaccretion 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
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| Ranked by size | ||
