Kuiper Belt Object (90482) Orcus is a trans-Neptunian object in the Kuiper belt with a large moon. It was discovered on February 17, 2004 by Michael Brown of Caltech, Chad Trujillo of the Gemini Observatory, and David Rabinowitz of Yale University. Precovery images as early as November 8, 1951 were later identified. It is thought to be "nearly certainly" a dwarf planet, although the IAU has not yet formally designated it as such. Read more
Brown and his team studied Orcus with both Hubble and Spitzer, two of the greatest observatories. Here are some facts about the Orcus system that are known reasonably precisely.
Vanth's orbit around Orcus. It's nearly circular, with a period of 9.5393 ± 0.0001 days at a distance of 8980 ± 20 kilometers.
The total mass of the Vanth-Orcus system. This follows directly from Vanth's orbital period, and is 6.32 ± 0.01 x 1020 kilograms, or, they report, about 3.8% the mass of Eris.
The brightness of Vanth with respect to Orcus. At visible wavelengths, Vanth is 2.54 ± 0.01 magnitudes fainter than Orcus.
An orbit of only 9000 kilometers is a really really close orbit. Still, Hubble is capable of resolving Orcus and Vanth as two separate (pointlike) objects. What they learned from looking at Orcus and Vanth with Hubble is that the two bodies have very different surfaces. Neither one shows the gray color and strong evidence of water ice that some other Kuiper belt bodies (like Haumea and Charon) do. There's some evidence for some water ice on Orcus, but Vanth looks like more like typical Kuiper belt bodies, reddish in color. It's not the color of Vanth that's odd; it's the fact that Vanth and Orcus have such different colors. In fact, Vanth and Orcus are less similar in color than any other binary pair yet observed in the Kuiper belt.
The name Vanth, the winged Etruscan psychopomp who guides the souls of the dead to the underworld, was chosen from among a large pool of submissions. Vanth was the only suggestion that was purely Etruscan in origin. It was the most popular submission, first suggested by Sonya Taaffe. This submission will be assessed by the IAU's Committee for Small Body Nomenclature, which will vote on whether to approve it, in accordance with the normal object naming procedures. Source