Hi'iaka

Hi'iaka is the outer and larger of Haumea's two known moons (as of August 2026). Haumea itself is a dwarf planet in the Kuiper Belt, orbiting the Sun at an average distance of about 43 astronomical units and taking about 285 Earth years per orbit. Hi'iaka was discovered on 26 January 2005 by Michael E. Brown's team using adaptive optics on the W. M. Keck telescope on Mauna Kea, Hawaii, and announced on 29 July 2005. It received its provisional designation S/2005 (2003 EL61) 1 and was formally named by the International Astronomical Union on 17 September 2008, alongside Haumea and Namaka. In Hawaiian tradition Hi'iaka is the patron goddess of hula and a daughter of Haumea, the goddess of fertility; the discovery team's informal nickname for the moon was Rudolph.

Its size is known only approximately. Recent estimates give a diameter of about 370 km, though older values of around 310 km are still widely quoted. Hi'iaka orbits Haumea at a mean distance of about 49,900 km with a period of about 49.5 days on a nearly circular path. Its mass is about 1.6 x 10^19 kg, roughly 0.5 per cent of Haumea's mass. Spectra show strong crystalline water-ice absorptions at 1.5 and 2.0 microns, so the surface is dominated by fresh water ice, like Haumea's own.

One point often misstated is that Hi'iaka, like most large moons, must be tidally locked. It is not: its light curve gives a rotation period of just under 10 hours, much shorter than its orbital period. Together with its icy composition, this supports the view that Hi'iaka and Namaka are fragments from an ancient collision that also produced Haumea's rapid 3.9-hour spin, its ring and its family of icy Kuiper Belt objects.

No spacecraft has visited Hi'iaka or Haumea, and none is planned. All detailed knowledge comes from ground-based telescopes, chiefly Keck, and from the Hubble Space Telescope, which helped refine the moons' orbits.