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50000 Quaoar (pronounced kwaa'·waar or kwow'·ər, Tongva ) * is a Trans-Neptunian object orbiting the Sun in the Edgeworth-Kuiper belt. It was discovered on June 4, 2002 by astronomers Chad Trujillo and Michael Brown at the California Institute of Technology in Pasadena, California from images acquired at the Samuel Oschin Telescope at Palomar Observatory. This discovery was announced on October 7, 2002, at a meeting of the American Astronomical Society. The earliest precovery turned out to be a May 25, 1954 plate from Palomar Observatory.

Size

Quaoar is estimated to have a diameter of 1260 ±190 km, which at the time of discovery made it the largest object found in the solar system since Pluto and, indeed, then the largest known minor planet (it was later supplanted by , , , Sedna and Orcus).

Quaoar was the first trans-Neptunian object to be measured directly from Hubble Space Telescope images, using a new sophisticated method (see Brown’s pages for a non-technical description and his paper cited in references for details). Given its distance Quaoar is on the limit of the HST resolution (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 HST optics (PSF), Brown and Troujillo were able to find the best fit disk's size which would give a similar blurred image. This method was recently applied by the same authors to measure the size of the biggest TNO so far: .

Quaoar's volume is somewhat more than all of the asteroids put together, it is about one tenth the diameter of Earth, one third the diameter of the Moon or about half the size of Pluto. It orbits at about 6 thousand million kilometres from the Sun with an orbital period of 287 years.

Name

The planetoid's name follows International Astronomical Union rules by naming all trans-Neptunian objects after creation deities (see planetary nomenclature). "Quaoar" is the name of a creation deity of the Native American Tongva people, native to the area around Los Angeles, where the discovery was named. The IAU approved the name Quaoar, making it the official name; it also has the provisional designation . The memorable number 50000 is a witness to the exciting race to discover a Pluto-sized object; Quaoar’s discovery followed that of 20000 Varuna and was in turn followed by bigger discoveries (see above).The efforts were finally rewarded with the discovery of an object bigger than Pluto: .

The planetoid's name is not to be confused with Qo'noS, the fictitious homeworld of the Klingon Empire in the Star Trek universe.

Orbit

Quaoar has a more typical planetary orbit than Pluto does—a near-circular, moderately-inclined (~8°) orbit with a radius of ~43 AU as shown on the polar view (Quaoar’s orbit in blue, Pluto’s in red, Neptune in grey). The spheres illustrate the current (April 2006) positions, relative sizes and colours. The perihelia (q), aphelia (Q) and the dates of passage are also marked.

Quaoar is classified as a classical trans-Neptunian object. Its cold orbit is however unusual for large classical objects that typically follow more eccentric and more inclined orbits (see cubewanos for the comparison and the definition of cold and hot families).

At 43 AU and a near-circular orbit, unlike Pluto which is in 2:3 orbital resonance with Neptune, Quaoar is not significantly perturbed by Neptune.

The ecliptic view illustrates the comparison of Quaoar' near-circular, moderately-inclined orbit with Pluto's highly inclined (~17°), highly eccentric (e=0.25) orbit.

Pluto's aphelion is beyond (and below) Quaoar's orbit, so that Pluto is closer to the Sun than Quaoar at some times of its orbit, and farther at others.

Physical characteristics

Quaoar is believed to be a mixture of rock and ice, like other Kuiper Belt Objects; however its very low albedo (estimated at 0.1 but still much higher than that of 20000 Varuna: 0.04) indicates that the ice has disappeared from its outer layers. The surface is moderately red (similar to Varuna and 28978 Ixion) in addition to being dim, unlike that of larger objects that are brighter (high albedo) and present neutral colour (see colour comparison).

In 2004, scientists were surprised to find signs of crystalline ice on Quaoar, indicating that the temperature rose to at least −160 °C (110 K or −260 °F) sometime in the last ten million years. Speculation began as to what could have caused Quaoar to heat up from its natural temperature of −220 °C (55 K or −360 °F). 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 (Jewitt & Luu, 2004).

Since then (2006), crystalline water ice was also found on but present in larger quantities and thought to be responsible for the very high albedo of that object (0.7).

If the New Horizons mission visits several Kuiper Belt Objects after visiting Pluto in 2015, our knowledge of the surfaces of small KBOs should improve but encounters with large objects seem unlikely.

References

External links

Trans-Neptunian objects

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This article is licensed under the GNU Free Documentation License. It uses material from the "50000 Quaoar".

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