50000 Quaoar - Size

Size

In 2004, Quaoar was estimated to have a diameter of 1260 ± 190 km, subsequently revised downward, which at the time of discovery in 2002 made it the largest object found in the Solar System since the discovery of Pluto. Quaoar was later supplanted by Eris, Sedna, Haumea, and Makemake. Quaoar is about as massive as (if somewhat smaller than) Pluto's moon Charon, which is approximately 2½ times as massive as Orcus. Quaoar is roughly one fifteenth the diameter of Earth, one quarter the diameter of the Moon, and a third the size of Pluto.

Quaoar was the first trans-Neptunian object to be measured directly from Hubble Space Telescope (HST) images, using a new, sophisticated method (see Brown’s pages for a non-technical description and his paper 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 (point spread function (PSF)), Brown and Trujillo were able to find the best-fit disk size which would give a similar blurred image. This method was recently applied by the same authors to measure the size of Eris.

The uncorrected 2004 HST estimates only marginally agree with the 2007 infrared measurements by the Spitzer Space Telescope which suggest a brighter albedo (0.19) and consequently a smaller diameter (844.4 +206.7
−189.6 km). During the 2004 HST observations, little was known about the surface properties of Kuiper belt objects, but we now know that the surface of Quaoar is in many ways similar to those of the icy satellites of Uranus and Neptune. Adopting a Uranian-satellite limb darkening profile suggests that the 2004 HST size estimate for Quaoar was approximately 40% too large, and that a more proper estimate would be about 900 km. Using a weighted average of the Spitzer and corrected HST estimates, Quaoar, as of 2010, can be estimated at about 890 ± 70 km in diameter.

On 2011-05-04 Quaoar occulted a 16th-magnitude star, which gave 1170 km as the longest chord and suggests an elongated shape. New measurement from Herschel Space Observatory with revised data from SST suggested that Quaoar has diameter 1070 ± 38 km and Weywot 81 ± 11 km.