Atlas · Orcus
The moon of Orcus
Orcus has one known moon, Vanth, found in 2005. Vanth is nearly half as wide as Orcus, so the two make a double world rather like Pluto and Charon. Its path is almost a perfect circle.
Orcus and its moon
Orcus and the orbit of its moon, to scale, seen from above the orbit.
The edge of the drawing is 11,500 km from the centre of Orcus.
How to read it
You are looking straight down on the orbit of Vanth, from its northern side. Which way Orcus itself is tipped is not well known, so its moon’s path is the surest thing to look down on. Orcus is drawn to scale with the orbits. The moons are not: at this scale most would be far too small to see, so each is a dot.
The drawing holds Orcus still at the centre. In truth the two circle a point between them.
- Solid dot. A spacecraft has mapped the whole surface.
- Half dot. A spacecraft has seen part of it.
- Ring. Never seen as more than a point of light.
- Pale line. Goes round the usual way, the way the planets go round the Sun: anticlockwise on these drawings.
The orbit is a slowly turning ellipse fitted to positions from NASA’s Jet Propulsion Laboratory for the years 2000 to 2030. The list gives how far out each fit can be. Touch a row to pick out its orbit.
The moon
1 moon
Vanth’s width was measured in 2017, when it passed in front of a star.
| Moon | Orbit size | One orbit | Tilt | Width | Found | See it in 3D |
|---|---|---|---|---|---|---|
| Vanth. Seen only as a point of light.drawn to within 0.28° | 8,975 km | 9.54 days | 90° | 443 km | 2005 | Fly there |
Where this comes from
- NASA/JPL Horizons: orbit solutions for the moons of trans-Neptunian objects, fitted here over 2000 to 2030
- Fornasier et al. 2013, Astronomy & Astrophysics 555, A15 (heat measured by the Herschel telescope)
- Sickafoose et al. 2019, Icarus 319, 657 (Vanth passing in front of a star)
- NASA/JPL Small-Body Database
Orbit size is the average distance from the centre of Orcus. Tilt is measured from the plane of the planets, and above 90° means the moon goes round backwards compared with the planets.