the system can be calculated from the orbit of the secondary. Quaoar's estimated density of around and estimated size of suggests that it is a dwarf planet. American astronomer Michael Brown estimates that rocky bodies around in diameter relax into hydrostatic equilibrium, and that icy bodies relax into hydrostatic equilibrium somewhere between and . With an estimated mass greater than , Quaoar has the mass and diameter "usually" required for being in hydrostatic equilibrium according to the 2006 IAU draft definition of a planet (5 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 to 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 Quaoar is thought to be an oblate spheroid around 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 and a measured equatorial diameter of , 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 with an uncertainty of , 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