Surface Properties
Hiʻiaka is the outer and, at roughly 350 km in diameter, the larger and brighter of the two moons. Strong absorption features observed at 1.5, 1.65 and 2 micrometres in its infrared spectrum are consistent with nearly pure crystalline water ice covering much of its surface. The unusual spectrum, and its similarity to absorption lines in the spectrum of Haumea, led Brown and colleagues to conclude that it was unlikely that the system of moons was formed by the gravitational capture of passing Kuiper belt objects into orbit around the dwarf planet: instead, the Haumean moons must be fragments of Haumea itself.
The sizes of both moons are calculated with the assumption that they have the same infrared albedo as Haumea, which is reasonable as their spectra show them to have the same surface composition. Haumea's albedo has been measured by the Spitzer Space Telescope: from ground-based telescopes, the moons are too small and close to Haumea to be seen independently. Based on this common albedo, the inner moon, Namaka, which is a tenth the mass of Hiʻiaka, would be about 170 km in diameter.
The Hubble Space Telescope (HST) has adequate angular resolution to separate the light from the moons from that of Haumea. Photometry of the Haumea triple system with HST's NICMOS camera has confirmed that the spectral line at 1.6 microns that indicates the presence of water ice is at least as strong in the moons' spectra as in Haumea's spectrum.
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.
Read more about this topic: Moons Of Haumea
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