500000 Quaoar

50000 Quaoar is a non-resonant trans-Neptunian object (cubewano) and a possible dwarf planet in the Kuiper belt, a region of icy planetesimals beyond Neptune. It measures approximately 1,121 km (697 mi) in diameter, about half the diameter of Pluto. The object was discovered by American astronomers Chad Trujillo and Michael Brown at the Palomar Observatory on 4 June 2002. Signs of water ice on the surface of Quaoar have been found, which suggests that cryovolcanism may be occurring on Quaoar. A small amount of methane is present on its surface, which can only be retained by the largest Kuiper belt objects. In February 2007, Weywot, a synchronous moon in orbit around Quaoar, was discovered by Brown. Weywot is measured to be 170 km (110 mi) across. Both objects were named after mythological figures from the Native American Tongva people in Southern California. Quaoar is the Tongva creator deity and Weywot is his son.

Discovery
Quaoar was discovered on 4 June 2002 by American astronomers Chad Trujillo and Michael Brown at the Palomar Observatory in the Palomar Mountain Range in San Diego County, California. The discovery formed part of the Caltech Wide Area Sky Survey, which was designed to search for the brightest Kuiper belt objects using the Palomar Observatory's 1.22-meter Samuel Oschin telescope. Quaoar was first identified in images by Trujillo on 5 June 2002, when he noticed a dim, 18.6-magnitude object slowly moving among the stars of the constellation Ophiuchus. Quaoar appeared relatively bright for a distant object, suggesting that it could have a size comparable to the diameter of the dwarf planet Pluto.

To ascertain Quaoar's orbit, Brown and Trujillo initiated a search for archival precovery images. They obtained several precovery images taken by the Near-Earth Asteroid Tracking survey from various observatories in 1996 and 2000–2002. In particular, they had also found two archival photographic plates taken by astronomer Charles T. Kowal in May 1983, who at the time was searching for the hypothesized Planet X at the Palomar Observatory. From these precovery images, Brown and Trujillo were able to calculate Quaoar's orbit and distance. Additional precovery images of Quaoar have been later identified, with the earliest known found by Dr. Edward Rhoads on a photographic plate imaged on 25 May 1954 from the Palomar Observatory Sky Survey.

Before announcing the discovery of Quaoar, Brown had planned to conduct follow-up observations using the Hubble Space Telescope to measure Quaoar's size. He had also planned to announce the discovery as soon as possible and found it necessary to keep the discovery information confidential during the follow-up observations. Rather than submitting his Hubble proposal under peer review, Brown submitted his proposal directly to one of Hubble's operators, who promptly allocated time to Brown. While setting up the observing algorithm for Hubble, Brown had also planned to use one of the Keck telescopes in Mauna Kea, Hawaii, as a part of a study on cryovolcanism on the moons of Uranus. This provided him additional time for follow-up observations and took advantage of the whole observing session in July to analyze Quaoar's spectrum and characterize its surface composition.

The discovery of Quaoar was formally announced by the Minor Planet Center in a Minor Planet Electronic Circular on 7 October 2002. It was given the provisional designation 2002 LM60, indicating that its discovery took place during the first half of June 2001. Quaoar was the 1,512nd object discovered in the first half of May, as indicated by the preceding letter and numbers in its provisional designation. On that same day, Trujillo and Brown reported their scientific results from observations of Quaoar at the 34th annual meeting of the American Astronomical Society's Division for Planetary Sciences in Birmingham, Alabama. They announced Quaoar was the largest Kuiper belt object found yet, surpassing previous record holders 20000 Varuna and 2002 AW197. Quaoar's discovery has been cited as Brown as having contributed to the reclassification of Pluto as a dwarf planet. Since then, Brown has contributed to the discovery of larger trans-Neptunian objects, including Haumea, Eris, Makemake, and Gonggong.

Naming
Upon Quaoar's discovery, it was initially given the temporary nickname "Object X" as a reference to Planet X, due to its potentially large size and unknown nature. At the time, Quaoar's size was uncertain, and its high brightness led the discovery team to speculate that it may be a possible tenth planet. After measuring Quaoar's size with the Hubble Space Telescope in July, the team began considering names for the object, particularly those from New World mythologies. Following the International Astronomical Union's (IAU's) naming convention for minor planets, non-resonant Kuiper belt objects are to be named after creation deities. The team settled on selecting names from Native American mythologies local to the Palomar Mountain region, the location where they discovered Quaoar. Through Internet searching, the team eventually decided upon the name Kwawar, the creator god of the Tongva people indigenous to the area around Los Angeles. The Tongva were the first inhabitants of the Los Angeles Basin, where Michael Brown's institute, the California Institute of Technology, is located.

According to Brown, the name "Quaoar" is pronounced with three syllables, and Trujillo's website on Quaoar gives a three-syllable pronunciation, /ˈkwɑː.oʊ(w)ɑːr/, as an approximation of the Tongva pronunciation [ˈkʷaʔuwar]. The name can be also pronounced as two syllables, /ˈkwɑːwɑːr/, reflecting the usual English spelling and pronunciation of the deity Kwawar.

In Tongva mythology, Kwawar (or Quaoar) is the creation force of the universe, singing and dancing deities into existence. Quaoar has no form or gender, although the deity is generally referred to with masculine pronouns. He first sings and dances to create Weywot (Sky Father), then they sing Chehooit (Earth Mother) and Tamit (Grandfather Sun) into existence. As they sang and danced deities into existence, the creation force became more complex as each joined the singing and dancing. Eventually, after reducing chaos to order, they created the great seven giants that upheld the world. They then gave rise to the lower animals and eventually the first man and woman, Tobohar and Pahavit.

Upon their investigation of names from Tongva mythology, Brown and Trujillo realized that there were existing members of the Tongva and considered contacting them for permission. They consulted tribal historian Marc Acuña, who suggested that the name Kwawar was appropriate for the newly-discovered object. The name was then changed to Qua-o-ar as this spelling was preferred among the Tongva. However, the name's spelling was later reconsidered by Brown due to its ambiguous pronunciation resulting from the four-vowel combination uaoa—no other word in the English language has this particular combination of vowels. (Indeed, even if one takes the u to be part of the initial consonant qu, no other word in English has the vowel sequence aoa, though it occurs in the place name Paraoa.) Regardless, the name and discovery of Quaoar were publicly announced in October, though Brown had not sought approval of the name by the IAU's Committee on Small Body Nomenclature (CSBN). Quaoar's name was announced before the official numbering of the object, which Brian Marsden—the head of the Minor Planet Center—remarked in 2004 to be a violation of the protocol during the time that the name of Sedna—another large trans-Neptunian object discovered by Brown—was announced. Despite this, the name was approved by the CSBN, and the naming citation, along with Quaoar's official numbering, was published in a Minor Planet Circular on 20 November 2002.

Quaoar was given the minor planet number 50000, which was not by coincidence but to commemorate its large size, being that it was found in the search for a Pluto-sized object in the Kuiper belt. The large Kuiper belt object 20000 Varuna has been similarly numbered for a similar occasion. However, subsequent even larger discoveries such as 136199 Eris were simply numbered according to the order in which their orbits were confirmed.

Physical characteristics
Quaoar's albedo or reflectivity could be as low as 0.1, similar to Varuna's albedo of 0.127. This may indicate that fresh ice has disappeared from Quaoar's surface. The surface is moderately red, meaning that Quaoar is relatively more reflective in the red and near-infrared spectrum than in the blue. The Kuiper belt objects Varuna and Ixion are also moderately red in the spectral class. Larger Kuiper belt objects are often much brighter because they are covered in more fresh ice and have a higher albedo, and thus they present a neutral color. A 2006 model of internal heating via radioactive decay suggested that, unlike 90482 Orcus, Quaoar may not be capable of sustaining an internal ocean of liquid water at the mantle–core boundary.

The presence of methane and other volatiles on Quaoar's surface suggest that it may support a tenuous atmosphere produced from the sublimation of volatiles. With a measured mean temperature of ~ 44 K (−229.2 °C), the upper limit of Quaoar's atmospheric pressure is expected to be in the range of a few microbars. Due to Quaoar's small size and mass, the possibility of Quaoar having an atmosphere of nitrogen and carbon monoxide has been ruled out, since the gases would escape from Quaoar. The possibility of a methane atmosphere, with the upper limit being less than 1 microbar, was considered until 2013, when Quaoar occulted a 15.8 magnitude star and revealed no sign of a substantial atmosphere, placing an upper limit to at least 20 nanobars, under the assumption that Quaoar's mean temperature is 42 K (−231.2 °C) and that its atmosphere consists of mostly methane. The upper limit of atmosphere pressure was tightened to 10 nanobars after another stellar occultation in 2019.

Mass and density
Because Quaoar is a binary object, the mass of the system can be calculated from the orbit of the secondary. Quaoar's estimated density of around 2.2 g/cm3 and estimated size of 1,121 km (697 mi) suggests that it is a dwarf planet. American astronomer Michael Brown estimates that rocky bodies around 900 km (560 mi) in diameter relax into hydrostatic equilibrium, and that icy bodies relax into hydrostatic equilibrium somewhere between 200 km (120 mi) and 400 km (250 mi). With an estimated mass greater than 1.6×1021 kg, Quaoar has the mass and diameter "usually" required for being in hydrostatic equilibrium according to the 2006 IAU draft definition of a planet (5×1020 kg, 800 km), and Brown states that Quaoar "must be" a dwarf planet. Light-curve-amplitude analysis shows only small deviations, suggesting that Quaoar is indeed a spheroid with small albedo spots and hence a dwarf planet.

Planetary scientist Erik Asphaug has suggested that Quaoar may have collided with a much larger body, stripping the lower-density mantle from Quaoar, and leaving behind the denser core. He envisioned that Quaoar was originally covered by a mantle of ice that made it 300 km (190 mi) to 500 km (310 mi) bigger than its present size, and that it collided with another Kuiper belt object about twice its size—an object roughly the diameter of Pluto, or even approaching the size of Mars. This model was made assuming Quaoar actually had a density of 4.2 g/cm3, but more recent estimates have given it a more Pluto-like density of only 2 g/cm3, with no further need for the collision theory.

Size
Hubble photo used to measure size of Quaoar

Quaoar compared to the Earth and the Moon.

Quaoar is thought to be an oblate spheroid around 1,121 km (697 mi) in diameter, being slightly flattened in shape. The estimates come from observations of stellar occultations by Quaoar, in which it passes in front of a star, in 2013 and 2019. Given that Quaoar has an estimated oblateness of 0.0897±0.006 and a measured equatorial diameter of 1138+48

−34 km, Quaoar is believed to be in hydrostatic equilibrium, being described as a Maclaurin spheroid. Quaoar is about as large and massive as (if somewhat smaller than) Pluto's moon Charon. Quaoar is roughly half the size of Pluto.

Quaoar was the first trans-Neptunian object to be measured directly from Hubble Space Telescope images, using a method comparing images with the Hubble point spread function (PSF). In 2004, Quaoar was estimated to have a diameter of 1,260 km (780 mi) with an uncertainty of 190 km (120 mi), using Hubble's measurements. Given its distance Quaoar is on the limit of Hubble's resolution of 40 milliarcseconds and its image is consequently "smeared" on a few adjacent pixels. By comparing carefully this image with the images of stars in the background and using a sophisticated model of Hubble optics (PSF), Brown and Trujillo were able to find the best-fit disk size that would give a similar blurred image. This method was also applied by the same authors to measure the size of the dwarf planet Eris.

At the time of its discovery in 2002, Quaoar was the largest object found in the Solar System since the discovery of Pluto. Quaoar's size was subsequently revised downward and was later superseded in size as larger objects (Eris, Haumea, Makemake and Gonggong) were discovered. The uncorrected 2004 Hubble estimates only marginally agree with the 2007 infrared measurements by the Spitzer Space Telescope that suggest a higher albedo (0.19) and consequently a smaller diameter (844.4+206.7

−189.6 km). Adopting a Uranian satellite limb darkening profile suggests that the 2004 Hubble size estimate for Quaoar was approximately 40 percent too large, and that a more proper estimate would be about 900 km. In 2010, Quaoar was estimated to be about 890 km in diameter, using a weighted average of Spitzer and corrected Hubble estimates. In observations of the object's shadow as it occulted an unnamed 16th-magnitude star on 4 May 2011, Quaoar was estimated to be 1,170 km (730 mi) in diameter. Measurements from the Herschel Space Observatory in 2013 suggested that Quaoar has a diameter of 1,070 km (660 mi). In that same year, Quaoar occulted a 15.8 magnitude star, yielding a chord length of 1100±5 km, consistent with the Herschel estimate. Another occultation by Quaoar in June 2019 also yielded a similar chord length of 1121±1.2 km.

Cryovolcanism
In 2004, signs of crystalline ice were found on Quaoar, indicating that the temperature rose to at least 110 K (−163 °C) sometime in the last ten million years. Speculation began as to what could have caused Quaoar to heat up from its natural temperature of 55 K (−218.2 °C). Some have theorized that a barrage of mini-meteors may have raised the temperature, but the most discussed theory speculates that cryovolcanism may be occurring, spurred by the decay of radioactive elements within Quaoar's core. Since then (2006), crystalline water ice was also found on Haumea, but present in larger quantities and thought to be responsible for the very high albedo of that object (0.7). More precise observations of Quaoar's near infrared spectrum in 2007 indicated the presence of small quantities (5%) of solid methane and ethane. Given its boiling point of 112 K (−161 °C), methane is a volatile ice at average surface temperatures of Quaoar, unlike water ice or ethane. Both models and observations suggest that only a few larger bodies (Pluto, Eris, and Makemake) can retain the volatile ices whereas the dominant population of small TNOs lost them. Quaoar, with only small amounts of methane, appears to be in an intermediary category.

Orbit and classification
Quaoar orbits the Sun at an average distance of 43.7 astronomical units (6.54×109 km; 4.06×109 mi), taking 288.8 years to complete one full orbit around the Sun. With an orbital eccentricity of 0.04, Quaoar follows a nearly circular orbit, only slightly varying in distance from 42 AU at perihelion to 45 AU at aphelion. At such distances, light from the Sun takes more than 5 hours to reach Quaoar. Quaoar has last passed aphelion in late 1932 and is currently approaching the Sun at a rate of 0.035 AU per year, or about 0.17 kilometers per second (380 mph). Quaoar will reach perihelion around February 2075.

Because Quaoar has a nearly circular orbit, it does not approach close to Neptune such that its orbit can become significantly perturbed under the gravitational influence of Neptune. Quaoar's minimum orbit intersection distance from Neptune is only 12.3 AU—it does not approach Neptune within this distance over the course of its orbit, as it is not in a mean-motion orbital resonance with Neptune. Simulations by the Deep Ecliptic Survey show that the perihelion and aphelion distances of Quaoar's orbit do not change significantly over the next 10 million years; Quaoar's orbit appears to be stable over the long term.

Quaoar is generally classified as a trans-Neptunian object or distant minor planet by the Minor Planet Center since it orbits in the outer Solar System beyond Neptune. Since Quaoar is not in a mean-motion resonance with Neptune, it is also classified as a classical Kuiper belt object (cubewano) by the Minor Planet Center and Deep Ecliptic Survey. Quaoar's orbit is moderately inclined to the ecliptic plane by 8 degrees, relatively high when compared to the inclinations of Kuiper belt objects within the dynamically cold population. Because Quaoar's orbital inclination is greater than 4 degrees, it is part of the dynamically hot population of high-inclination classical Kuiper belt objects. The high inclinations of hot classical Kuiper belt objects such as Quaoar are thought to have resulted from gravitational scattering by Neptune during its outward migration in the early Solar System.

Rotation
Quaoar's rotation period is uncertain, and two possible rotation periods of Quaoar are given (8.64 hours or 17.68 hours). Derived from the rotational light curves of Quaoar observed on March through June 2003, its rotation period is measured to be 17.6788 hours.

Satellite
Quaoar has one known moon, Weywot (full designation (50000) Quaoar I Weywot), discovered in 2006. It is thought to be somewhere around 170 km (110 mi) in diameter.

Exploration
It was calculated that a flyby mission to Quaoar could take 13.57 years using a Jupiter gravity assist, based on launch dates of 25 December 2016, 22 November 2027, 22 December 2028, 22 January 2030 or 20 December 2040. Quaoar would be 41 to 43 AU from the Sun when the spacecraft arrives. In July 2016, the Long Range Reconnaissance Imager (LORRI) aboard the New Horizons spacecraft took a sequence of four images of Quaoar from a distance of about 14 AU. Pontus Brandt at Johns Hopkins Applied Physics Laboratory and his colleagues have studied an interstellar probe that would potentially fly by Quaoar in the 2030s before continuing to the interstellar medium. Quaoar has been chosen as a flyby target for such a mission particularly for its escaping methane atmosphere and possible cryovolcanism, as well as its close proximity to the heliospheric nose.