Hiiaka, formal designation (136108) Haumea I, is the larger, outer moon of the trans-Neptunian

dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit around the Sun, massive enough to be hydrostatic equilibrium, gravitationally rounded, but insufficient to achieve clearing the neighbourhood, orbital dominance like the ...

Haumea Haumea ( minor-planet designation: 136108 Haumea) is a dwarf planet located beyond Neptune's orbit. It was discovered in 2004 by a team headed by Mike Brown of Caltech at the Palomar Observatory, and formally announced in 2005 by a team heade ...

. It is named after one of the daughters of

, Hiiaka, the patron goddess of the Big Island of Hawaii. It orbits once every at a distance of , with an eccentricity of and an inclination of . Assuming its estimated diameter of over 300 km is accurate, it may be the fourth- or fifth-largest known moon of a Trans-Neptunian object, after Pluto I Charon, Eris I Dysnomia, Orcus I Vanth, and possibly Varda I Ilmarë and Salacia I Actaea.

Discovery

Hiiaka was the first

satellite A satellite or an artificial satellite is an object, typically a spacecraft, placed into orbit around a celestial body. They have a variety of uses, including communication relay, weather forecasting, navigation ( GPS), broadcasting, scient ...

discovered around Haumea. It was discovered on 26 January 2005 and nicknamed " Rudolph" by the discovery team before being assigned an official name.

Physical characteristics

Size and brightness

Stellar occultation Stellar means anything related to one or more stars (''stella''). The term may also refer to: Arts, entertainment, and media * Stellar (magazine), ''Stellar'' (magazine), an Irish lifestyle and fashion magazine * Stellar Loussier, a character fro ...

s by Hiiaka on 6 and 16 April 2021 reveal that the moon is elongated, with estimated ellipsoid dimensions of . These correspond to a volume-equivalent diameter of . To put this in perspective, if Hiiaka were in the asteroid belt, it would be larger than all but the four largest asteroids, after

1 Ceres Ceres ( minor-planet designation: 1 Ceres) is a dwarf planet in the middle main asteroid belt between the orbits of Mars and Jupiter. It was the first known asteroid, discovered on 1 January 1801 by Giuseppe Piazzi at Palermo Astronomical O ...

2 Pallas Pallas (minor-planet designation: 2 Pallas) is the List of largest asteroids, third-largest asteroid in the Solar System by volume and mass. It is the second asteroid to have been discovered, after 1 Ceres, Ceres, and is likely a remnant ...

4 Vesta Vesta (minor-planet designation: 4 Vesta) is one of the largest objects in the asteroid belt, with a mean diameter of . It was discovered by the German astronomer Heinrich Wilhelm Matthias Olbers on 29 March 1807 and is named after Vesta (mytho ...

, and 10 Hygiea. In spite of its relatively large size, Hiiaka is not in

hydrostatic equilibrium In fluid mechanics, hydrostatic equilibrium, also called hydrostatic balance and hydrostasy, is the condition of a fluid or plastic solid at rest, which occurs when external forces, such as gravity, are balanced by a pressure-gradient force. I ...

because its elongated shape is inconsistent with that expected for its current rotation period.

Mass and density

The mass of Hiiaka is estimated to be using precise relative

astrometry Astrometry is a branch of astronomy that involves precise measurements of the positions and movements of stars and other Astronomical object, celestial bodies. It provides the kinematics and physical origin of the Solar System and this galaxy, th ...

from the

Hubble Space Telescope The Hubble Space Telescope (HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the Orbiting Solar Observatory, first space telescope, but it is one of the largest and most ...

and Keck Telescope and applying 3-body, point-mass model to the Haumean system. Hiiaka's diameter and mass indicate it has a low density of , which suggests Hiiaka's interior consists of highly porous water ice with a rock mass fraction between 50% and 70%.

Spectrum and composition

The near infrared spectrum of Hiiaka is dominated by water-ice absorption bands, which means that its surface is made mainly of water ice. The presence of the band centered at indicates that the surface water ice is primarily in the

crystalline A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macrosc ...

form. Currently it is unclear why water ice on the surface has not turned into

amorphous In condensed matter physics and materials science, an amorphous solid (or non-crystalline solid) is a solid that lacks the long-range order that is a characteristic of a crystal. The terms "glass" and "glassy solid" are sometimes used synonymousl ...

form as would be expected due to its constant irradiation by

cosmic ray Cosmic rays or astroparticles are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the ...

References

External links

Brown's publication describing the discovery of Hiiaka

Paper describing the composition of Hiiaka
{{DEFAULTSORT:Hi'iaka (moon) Moons of Haumea Trans-Neptunian satellites Discoveries by Michael E. Brown Discoveries by Chad Trujillo Discoveries by David L. Rabinowitz Objects observed by stellar occultation