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Moons of Haumea

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File:2003 EL61.jpg
Keck image of Haumea and its moons. Hiʻiaka is above Haumea (center), and Namaka is directly below.

The dwarf planet Haumea has two known moons, Hiʻiaka and Namaka, named after deities from Hawaiian mythology. Both of these small moons were discovered in 2005, from observations of Haumea made at the large telescopes of the W. M. Keck Observatory in Hawaii.

Haumea's moons are unusual in several ways. They are considered to be part of its extended collisional family, which formed billions of years ago from icy debris after a large impact disrupted Haumea's ice mantle. Hiʻiaka, the larger, outermost moon, has large amounts of pure water ice on its surface, a feature rare among Kuiper belt objects.[1] Namaka, about ten times smaller, has an orbit with surprising dynamics: it is unusually eccentric and appears to be greatly influenced by the larger satellite.

Discovery and naming

Two small satellites were discovered around Haumea (which was at that time still designated 2003 EL61) through observations using the W.M. Keck Observatory by a Caltech team in 2005. The outer and larger of the two satellites was discovered January 26, 2005,[2] and formally designated S/2005 (2003 EL61) 1, though nicknamed "Rudolph" by the Caltech team.[3] The smaller, inner satellite of Haumea was discovered on June 30, 2005, formally termed S/2005 (2003 EL61) 2, and nicknamed "Blitzen".[4] On September 7, 2006, both satellites were numbered and admitted into the official minor planet catalogue as (136108) 2003 EL61 I and II, respectively.

The permanent names of these moons were announced, together with that of 2003 EL61, by the International Astronomical Union on September 17, 2008: (136108) Haumea I Hiʻiaka and (136108) Haumea II Namaka.[5] Each moon was named after a daughter of Haumea, the Hawaiian goddess of fertility and childbirth. Hiʻiaka is the goddess of dance and patroness of the Big Island of Hawaii, where the Mauna Kea Observatory is located.[6] Nāmaka is the goddess of water and the sea; she cooled her sister Pele's lava as it flowed into the sea, turning it into new land.[7]

In her legend, Haumea's many children came from different parts of her body.[7] The dwarf planet Haumea appears to be almost entirely made of rock, with only a superficial layer of ice; most of the original icy mantle is thought to have been blasted off by the impact that spun Haumea into its current high speed of rotation, where the material formed into the small Kuiper belt objects in Haumea's collisional family. There could therefore be a large number of moons smaller than Namaka and below current detection levels.[8]

Surface and orbital characteristics

Hiʻiaka is the outer and, at roughly 310 km in diameter, the larger and brighter of the two moons, and orbits Haumea in a nearly circular path every 49 days.[9] Strong absorption features observed at 1.5 and 2 micrometres in the infrared spectrum are consistent with nearly pure crystalline water ice covering much of its surface.[1] The unusual spectrum, along with similar absorption lines on Haumea, led Brown and colleagues to conclude that capture by the dwarf planet of passing objects into the present orbits was an unlikely model for the system's formation, and that the Haumean moons must be fragments of Haumea itself.[10]

The sizes of both moons are calculated with the assumption that they have the same albedo as Haumea. Albedo has been measured for the dwarf planet by the Spitzer Space Telescope, but it cannot be measured directly for the moons, as they are too small and close to Haumea to be seen independently. Based on this common albedo, the inner moon, Namaka, would be about 170 km in diameter.[11]

Namaka is a tenth the mass of Hiʻiaka, orbits Haumea in 18 days in a highly elliptical, non-Keplerian orbit, and as of 2008 is inclined 13° from the larger moon, which perturbs its orbit.[4] Since the impact that created the moons of Haumea is thought to have occurred in the early history of the Solar System,[12] over the following billions of years it should have been tidally damped into a more circular orbit. Current research suggests that Namaka's orbit has been disturbed by orbital resonances with the more massive Hiʻiaka, due to converging orbits as the two moons move outward from Haumea due to tidal dissipation.[4] The moons may have been caught in and then escaped from orbital resonance several times; they currently are in or at least close to an 8:3 resonance.[4] This resonance strongly perturbs Namaka's orbit, which has a current precession of ~20°.[4]

The moons of Haumea are too faint to detect with telescopes smaller than about 2 metres in aperture, though Haumea itself has a visual magnitude of 17.5, making it the third brightest object in the Kuiper belt after Pluto and Makemake, and easily observable with a large amateur telescope.

At present, the orbits of the Haumean moons appear almost exactly edge-on from Earth, with the moons potentially occulting Haumea.[13] Observation of such transits would provide precise information on the size and shape of Haumea and its moons, as happened in the late 1980s with Pluto and Charon.[14] The tiny change in brightness of the system during these occultations will require at least a medium-aperture professional telescope for detection.[15] Hiʻiaka last occulted Haumea in 1999, a few years before discovery, and will not do so again for some 130 years.[16] However, in a situation unique among regular satellites, the great torquing of Namaka's orbit by Hiʻiaka will preserve the viewing angle of Namaka–Haumea transits for several more years.[4][15]

Name
(pronunciation) 		Mean diameter
(km) 		Mass
(×1021 kg) 		Semi-major
axis (km) 		Orbital period
(days) 		Eccentricity Inclination Discovery date
Haumea II Namaka /nɑːˈmɑːkə/ ~170 km? ~0.002
(0.05% Haumea)		 observed at
39 000		 18 0.23[17] 13° from Hiʻiaka[17] December 2005
Haumea I Hiʻiaka /hiːʔiːˈɑːkə/ ~310 km ~0.02
(0.5% Haumea)		 49 500 ± 400 49.12 ± 0.03 0.050 ± 0.003 234.8 ±0.3° June 2005

See also

References

  1. ^ a b K. M Barkume, M. E. Brown, and E. L. Schaller (March 2006). preprint "Water Ice on the Satellite of Kuiper Belt Object 2003 EL61". The Astrophysical Journal. 640: L87–L89. doi:10.1086/503159. {{cite journal}}: Check |url= value (help)CS1 maint: multiple names: authors list (link) Cite error: The named reference "Barkume2006" was defined multiple times with different content (see the help page).
  2. ^ M. E. Brown, A. H. Bouchez, D. Rabinowitz. R. Sari, C. A. Trujillo, M. van Dam, R. Campbell, J. Chin, S. Hardman, E. Johansson, R. Lafon, D. Le Mignant, P. Stomski, D. Summers, and P. Wizinowich (2005-09-02). "Keck Observatory Laser Guide Star Adaptive Optics Discovery and Characterization of a Satellite to the Large Kuiper Belt Object 2003 EL61". The Astrophysical Journal Letters. 632: L45–L48. doi:10.1086/497641.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  3. ^ Kenneth Chang (2007-03-20). "Piecing Together the Clues of an Old Collision, Iceball by Iceball". New York Times. Retrieved 2008-10-12.
  4. ^ a b c d e f D. Ragozzine, M. E. Brown, C. A. Trujillo, E. L. Schaller. "Orbits and Masses of the 2003 EL61 Satellite System". AAS DPS conference 2008. Retrieved 2008-10-17.{{cite web}}: CS1 maint: multiple names: authors list (link)
  5. ^ "News Release - IAU0807: IAU names fifth dwarf planet Haumea". International Astronomical Union. 2008-09-17. Retrieved 2008-09-18.
  6. ^ "Dwarf Planets and their Systems". US Geological Survey Gazetteer of Planetary Nomenclature. Retrieved 2008-09-17.
  7. ^ a b Robert D. Craig (2004). Handbook of Polynesian Mythology. ABC-CLIO. p. 128.
  8. ^ "The Haumean system as a collisional family". SPACE.com. 2007-03-14. Retrieved 2009-02-01.
  9. ^ M. E. Brown, M. A. van Dam, A. H. Bouchez; et al. (2005-10-02). "Satellites of the largest Kuiper belt objects" (PDF). The Astrophysical Journal. 639: 43–46. doi:10.1086/501524. Retrieved 2009-09-29. {{cite journal}}: Explicit use of et al. in: |author= (help)CS1 maint: multiple names: authors list (link)
  10. ^ Michael E. Brown. "The largest Kuiper belt objects" (PDF). CalTech. Retrieved 2008-09-19.
  11. ^ "(136108) Haumea, Hi'iaka, and Namaka". Johnstonsarchive.net. Retrieved 2009-02-01.
  12. ^ Michael E. Brown, Kristina M. Barkume; Darin Ragozzine; Emily L. Schaller (2007-01-19). "A collisional family of icy objects in the Kuiper belt". Nature. 446 (7133): 294–296. doi:10.1038/nature05619. {{cite journal}}: |access-date= requires |url= (help)CS1 maint: multiple names: authors list (link)
  13. ^ "IAU Circular 8949". International Astronomical Union. 2008-09-17. Retrieved 2008-12-06.
  14. ^ Lucy-Ann Adams McFadden, Paul Robert Weissman, Torrence V. Johnson. "Encyclopedia of the Solar System". Retrieved 2008-10-17.{{cite web}}: CS1 maint: multiple names: authors list (link)
  15. ^ a b D. C. Fabrycky, M. J. Holman, D. Ragozzine, M. E. Brown, T. A. Lister, D. M. Terndrup, J. Djordjevic, E. F. Young, L. A. Young, R. R. Howell. "Mutual Events of 2003 EL61 and its Inner Satellite". AAS DPS conference 2008. Retrieved 2008-10-17.{{cite web}}: CS1 maint: multiple names: authors list (link)
  16. ^ Mike Brown (2008-05-18). "Moon shadow Monday (fixed)". Mike Brown's Planets. Retrieved 2008-09-27.
  17. ^ a b In 2008; Namaka's eccentricity and inclination are variable due to perturbation.
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