(provisional designation ) is an icy trans-Neptunian object orbiting the Sun in the Kuiper belt. It has a diameter of about and a highly reflective surface made of fresh water ice. It is the brightest and possibly the largest known member of the Haumea family, a population of Kuiper belt objects that broke off from the dwarf planet Haumea 4.4 billion years ago.
was discovered on 15 October 2002 by the NASA-directed Near-Earth Asteroid Tracking (NEAT) survey at Palomar Observatory. When it was discovered, astronomers initially inferred from its high brightness that it could be a large dwarf planet almost in diameter. This was later disproven in October 2009, when astronomers measured the diameter of for the first time by observing the object occulting or blocking out the light of a background star. is now known to be too small to likely qualify as a dwarf planet, and its brightness mainly comes from its reflective surface. was the first Kuiper belt object (besides Pluto and its moon Charon) that was observed via stellar occultation.
was discovered on 15 October 2002 by the Near-Earth Asteroid Tracking (NEAT) survey, which was a NASA-directed project for finding near-Earth asteroids in the sky using telescopes at various observatories across the United States. The telescope that discovered was the Samuel Oschin telescope at Palomar Observatory in San Diego County, California. The people involved in making the discovery observations at Palomar included Eleanor Helin, Steven Pravdo, Kenneth Lawrence, Michael D. Hicks, and R. Thicksten. Additional confirming observations were provided by Powell Observatory, Table Mountain Observatory, and Stephen P. Laurie at Church Stretton (obs. code 966) during the three days following the discovery of .
The Minor Planet Center (MPC) announced the discovery of on 22 Oct 2002. Within a day after the announcement, astronomer Reiner Stoss found pre-discovery observations of in NEAT images from 2001 to 2002 and Digitized Sky Survey images from 1954 to 1995. The earliest known pre-discovery observation of came from Siding Spring Observatory images taken on 27 August 1954.
This object has the minor planet provisional designation , which was given by the MPC in the discovery announcement. The provisional designation indicates the year and half-month of the object's discovery date. received its permanent minor planet catalog number of 55636 from the MPC on 16 February 2003. The Kuiper belt objects 55637 Uni and directly come after 's number in the minor planet catalog.
does not have a proper name and the discoverers' privilege for naming this object expired ten years after it was numbered. According to naming guidelines by the International Astronomical Union's Working Group for Small Bodies Nomenclature, is open for name suggestions that relate to creation myths, as recommended for Kuiper belt objects in general.
is a trans-Neptunian object (TNO) orbiting the Sun at a semi-major axis or average distance of 43.3 astronomical units (AU). It follows an elliptical orbit that has an eccentricity of 0.12 and a high inclination of 25.9ð with respect to the ecliptic. In its 284-year-long orbit, comes as close as 37.9 AU from the Sun at perihelion and as far as 48.6 AU from the Sun at aphelion. last passed perihelion in February 1952 and will make its next perihelion passage on 24 June 2242.
is located in the classical region of the Kuiper belt 39âÂÂ48 AU from the Sun, and is thus classified as a classical Kuiper belt object (sometimes known as a "cubewano"). The high orbital inclination of makes it a dynamically "hot" member of the classical Kuiper belt, which implies that it was gravitationally scattered out to its present location by Neptune's outward planetary migration in the Solar System's early history. Because of this, has sometimes been classified as a "scattered" object.
The orbit of is very similar to that of the dwarf planet Haumea, which suggests that could be a collisional fragment of the dwarf planet. 's surface color, composition, and reflectivity matches that of Haumea as well, which lends further credibility to this scenario. It is now widely accepted among researchers that originated from Haumea; it is classified as a member of the Haumea family, a population of Kuiper belt objects that are suspected to be fragments of Haumea.
The surface of is mostly made of highly reflective fresh water ice, which makes the object appear bright and "spectrally neutral" in visible lightâÂÂmeaning it reflects equal amounts of light across the visible spectrum, which gives a gray color. With a geometric albedo between 50% and 80%, has one of the most reflective surfaces seen among Kuiper belt objects (KBOs). stands out from typical KBOs, whose surfaces are commonly dark and reddened due to irradiated organic compounds or tholins.
The object's spectrum in the visible and near-infrared ranges is very similar to that of Charon, characterized by neutral to blue slope (1%/1000 à) with deep (60%) water absorption bands at 1.5 and 2.0 üm. Mineralogical analysis indicates a substantial fraction of large ice (H<sub>2</sub>O) particles. The signal-to-noise ratio of the observations was insufficient to differentiate between amorphous or crystalline ice (crystalline ice was reported on Charon, Quaoar and Haumea). The proportion of highly processed organic materials (tholins), typically present on numerous trans-Neptunian objects, is very low. As suggested by Licandro et al. 2006, this lack of irradiated mantle suggest either a recent collision or comet activity.
has a diameter of , according to measurements from stellar occultation observations. At this size, is too small to gravitationally collapse itself into a sphere (be in hydrostatic equilibrium), so it does not qualify as a dwarf planet. For comparison, the diameter of Saturn's smallest round moon Mimas is . Although the mass and density of have not been measured, its overall composition has been inferred to be mostly water ice like its surface, which would suggest that 's bulk density should be likely around , similar to that of water ice. This density would correspond to a mass on the order of (about times the mass of Earth's Moon) for .
When was discovered, astronomers initially thought it could be a large dwarf planet up to in diameter, because it was one of the brightest KBOs known at the time. was even included in the International Astronomical Union's (IAU) roster of dwarf planet candidates in their first draft of the 2006 IAU definition of "planet", where they cited an upper limit diameter of for . This overestimation of 's diameter came from astronomers incorrectly assuming that it has a dark, low-albedo surface like the bright KBOs 50000 Quaoar, 55565 Aya, and 20000 Varuna. Astronomers began suspecting that must be smaller than initially thought, when far-infrared and radio telescopes showed no significant thermal emission coming from the object.
Astronomers were able to measure the diameter and albedo of for the first time on 9 October 2009, when the object occulted or blocked out the light of a background star. was the first Kuiper belt object (besides Pluto and its moon Charon) that was observed via stellar occultation. The stellar occultation was predicted and observed by a network of astronomers stationed at 18 different locations across Hawaii. Only two of these locations successfully observed the occultation; an initial analysis of their detections indicated that should have a diameter of if it was spherical. However, this diameter estimate was affected by a timing error between the two occultation detections; reanalysis of the 2009 occultation data in 2018 and 2019 suggested that should have a slightly larger diameter of about .
Although the exact shape of is unknown, it is expected to be similar to an oblate spheroid, with irregularities. Observations of 's brightness over time indicate it has a rotation period of either 8.04 or 16.08 hours, depending on whether the object's brightness variability is caused by surface albedo variations or an elongated shape. Studies from 2003 to 2018 have consistently shown that exhibits very little brightness variation (less than 0.1 magnitudes), which makes it difficult to determine its rotation period. The axial tilt of is unknown.
Observations of the 9 October 2009 stellar occultation by showed that the object lacks an atmosphere with a surface number density greater than particles per cubic centimetre. is expected to not have an atmosphere because its mass is too small for its gravity to hold onto light gases.