Vanth (moon)

Vanth (formal designation (90482) Orcus I) is a natural satellite or moon of the large trans-Neptunian dwarf planet Orcus. It was discovered by Michael Brown and Terry-Ann Suer using images taken by the Hubble Space Telescope on 13 November 2005. The moon has a diameter of 443 km (275 mi), making it about half the size of Orcus and the third-largest moon of a trans-Neptunian object. Vanth is massive enough that it shifts the barycenter of the Orcus–Vanth system outside of Orcus, forming a binary system in which the two bodies revolve around the barycenter, much like the Pluto–Charon system. It is hypothesized that both systems formed similarly, most likely by a giant impact early in the Solar System's history. Compared to Orcus, Vanth has a darker and slightly redder surface that supposedly lacks exposed water ice, resembling primordial Kuiper belt objects.

Vanth was discovered in Hubble Space Telescope images taken on 13 November 2005, during Michael Brown's survey for satellites around large trans-Neptunian objects (TNOs) using Hubble's high-resolution Advanced Camera for Surveys. After Brown's Hubble survey concluded in late 2006, he and his colleague Terry-Ann Suer reported their newly discovered TNO satellites to the Central Bureau for Astronomical Telegrams, which announced their discovery of Vanth alongside three other TNO satellites on 22 February 2007. Brown continued observing the Orcus–Vanth system with Hubble in October–December 2006 and November–December 2007 to better determine the moon's orbit.

Before Vanth was named, it did not have a provisional designation. On 23 March 2009, Brown asked readers of his blog to suggest possible names for the satellite, with the best one to be submitted to the International Astronomical Union (IAU) on 5 April. The name Vanth, the winged Etruscan psychopomp who guides the souls of the dead to the underworld, was first suggested by Sonya Taaffe—a fiction writer—and became the most popular name among the large pool of suggestions.

Vanth and Persipnei were among the few names that both matched the Etruscan origin and chthonic theme of Orcus's name, though Brown ultimately chose Vanth because its relationship to Orcus in Etruscan mythology strongly parallels the relationship between Pluto and Charon in Greek mythology. In Etruscan iconography, Vanth is frequently portrayed in the company of Charun (the Etruscan counterpart of the Greek Charon), which alludes to the similar properties of the Pluto and Orcus systems (the latter being nicknamed the "anti-Pluto" because the orbital resonance with Neptune keeps it on the opposite side of the Sun from Pluto). Brown quoted Taaffe as saying that if Vanth "accompanies dead souls from the moment of death to the underworld itself, then of course her face is turned always toward Orcus", a reference to the likely synchronous orbit of Vanth about Orcus.

The submission for Vanth's name was assessed and approved by the IAU's Committee for Small Body Nomenclature, in agreement with the naming procedures for minor planets and satellites. The official naming citation was announced by the Minor Planet Center in a notice published on 30 March 2010.

From Earth, Vanth appears very close to Orcus with an angular separation of up to 0.25 arcseconds. For this reason, Vanth can only be visually resolved in high-resolution imaging, which requires the use of large-aperture space telescopes or ground-based telescopes aided by adaptive optics or interferometry. In visible light, Vanth's apparent magnitude is about 22, which is 2.61 magnitudes fainter than Orcus or about 9% of Orcus's brightness. Orcus and Vanth will gradually brighten as the system draws closer to the Sun until perihelion in 2142.

Stellar occultations are a useful way of directly measuring an object's position, size, and shape, and can be predicted when the object's orbital trajectory is well-known. The first successful detection of a stellar occultation by Vanth was made by a single observatory in Hokkaido, Japan on 1 March 2014, which detected the occultation lasting 3 seconds. Because this was only a single detection of the occulted star's chord across Vanth, the occultation did not provide a meaningful constraint on Vanth's diameter and shape. On 7 March 2017, another stellar occultation by Vanth was observed in the Americas and the Pacific Ocean. Of the five observatories that participated in observing the 2017 occultation by Vanth, two of them made positive detections. The remaining observatories that did not detect the occultation, alongside the fact that the occulted star was a double star, tightly constrained the range of Vanth's possible diameters to 432–453 km (268–281 mi), with the assumption that Vanth had a spherical shape. The 2017 occultation showed no signs of an atmosphere on Vanth, which places an upper bound pressure of 1–4 microbars for a potential atmosphere. The 2017 occultation also showed no signs of rings within 10,000 km (6,200 mi) from Vanth or beyond 8,010 km (4,980 mi) from Orcus.

Vanth forms a binary system with Orcus, in which the two bodies revolve around the barycenter between them. Orcus and Vanth are separated 9,000 km (5,600 mi) apart from each other's centers and revolve around their barycenter in nearly circular orbits with a period of 9.54 days. Vanth is less massive than Orcus, so it is the secondary component of the binary system and it orbits farther out from the barycenter at an orbital radius of 7,770 km (4,830 mi; 86.3% of the Orcus–Vanth separation distance). The more massive primary component, Orcus, orbits closer to the barycenter at an orbital radius of 1,230 km (760 mi; 13.7% of the separation distance).