Moons of outer solar system - Vanth (moon of Orcus)

Vanth, officially (90482) Orcus I Vanth, is the single known natural satellite of the plutino and likely dwarf planet Orcus.

Vanth is the 23th largest moon in the Solar System currently known.

Discovery

Vanth was discovered by Mike Brown and T.-A. Suer using discovery images taken by the Hubble Space Telescope on November 13, 2005. The discovery was announced on 22 February, 2007 in IAUC 8812.

Naming

Upon discovery, Vanth was issued a provisional designation, S/2005 (90482) 1. On March 23, 2009, Brown asked readers of his weekly column to suggest possible names for the satellite, with the best one to be submitted to the International Astronomical Union (IAU) on April 5.

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.

The Etruscan Vanth is frequently portrayed in the company of Charun (Charon), and so as the name of the moon of Orcus (nicknamed the "anti-Pluto" because resonance with Neptune keeps it on the opposite side of the Sun from Pluto), it is an allusion to the parallels between Orcus and Pluto.

Stats

Diameter (estimated): 280 km (280 - 380 km)

Semi-major axis: 9030 ± 89 km

Orbital Period: 9.54 days

Rotation Period: Synchronous

Orbit

Vanth orbits Orcus "in a tight precise circle", with a period of approximately 10 days.

Michael E. Brown suspects that, like Pluto and Charon, Orcus and Vanth are tidally locked.

Formation

The satellite does not resemble other known collisional satellites because its spectrum is very different of that of primary and thus may be a captured KBO.

Physical Properties

Vanth was found at 0.25 arcsec from Orcus with magnitude difference of 2.7 ± 1.0. Estimates made in 2009 by Mike Brown show that the apparent magnitude of Vanth is 21.97 ± 0.05 which is 2.54 ± 0.01 magnitudes fainter than Orcus. Assuming equal albedos this would mean a diameter of 280 km, or 2.9 times smaller than the primary.

However, the dissimilar colors of Orcus (neutral) and Vanth (red) suggest that Vanth could have an albedo a factor of two lower than Orcus. Should Vanth have an albedo of only 0.12, Vanth could be as large as 380 km with Orcus being 760 km in diameter. The mass of Vanth also depends on its albedo and can vary from 3 to 9% of the total system mass.