Orcus
Orcus is a dwarf planet locked in the same 3:2 orbital resonance with Neptune as Pluto, but always on the opposite side of its orbit — earning it the nickname 'anti-Pluto'. It has a large moon, Vanth. No spacecraft has visited.
Key facts
Science
Physical
- Mass
- 6.30 × 10²⁰ kg · 0.00858 Moon masses
- Radius
- 460 km · 7% of Earth's radius
- Density
- 1,545 kg/m³ · 1.5× water
- Computed from mass and radius
- Surface gravity
- 0.199 m/s² · 2% of Earth's gravity
- Computed from mass and radius
- Escape velocity
- 428 m/s · 4% of Earth's escape velocity
- Computed from mass and radius
Orbital
- Distance from the Sun
- 48 AU
- Computed from the position at the selected time
- Semi-major axis
- 39.4 AU
- Computed from the cached state vector
- Eccentricity
- 0.22
- Computed from the cached state vector
- Inclination
- 30.36 °
- Computed from the cached state vector
- Orbital period
- 247.27 years
- Computed from the cached state vector
- Perihelion
- 30.7 AU
- Computed from the orbital elements
- Aphelion
- 48.1 AU
- Computed from the orbital elements
- Mean orbital velocity
- 4.75 km/s
- Computed from the orbital elements
Rotation
- Rotation period
- 13.19 hours
Composition
- Bulk composition
- Spectroscopy shows crystalline water ice and probably ammonia on a fairly neutral-coloured surface; both are slowly destroyed by radiation, hinting at relatively recent cryovolcanic resurfacing. The deep interior is unconstrained — no spacecraft has visited.
- Surface imagery
- No real surface imagery exists — this object has never been visited; color is approximate, inferred from spectroscopy.
Radiation & temperature
- Insolation
- 0.000644 S⊕ · 1/1,552 of the sunlight Earth receives
- Computed from the distance to the Sun (inverse-square; Earth's sunlight = 1), before any atmosphere.
- Geometric albedo
- 0.23
- V-band geometric albedo ≈ 0.23; Bond albedo not directly measured.
- Equilibrium temperature
- 42 K (-232 °C)
- Computed from albedo + distance from the Sun (airless black body); uses the geometric albedo as an estimate
Discovery & history
- Discovered
- February 17, 2004
- By
- Mike Brown, Chad Trujillo and David Rabinowitz
- Method
- Photographic plates
- Name
- Orcus, a god of the underworld in Etruscan and Roman myth — a deliberate counterpart to Pluto, whose orbit Orcus mirrors; its moon Vanth is an Etruscan underworld spirit.
Orcus was discovered on 17 February 2004 by Mike Brown, Chad Trujillo and David Rabinowitz (provisional designation 2004 DW). It follows the same 3:2 resonance with Neptune as Pluto, but always sits at the opposite point of its orbit — earning it the nickname the 'anti-Pluto', which guided its naming after Orcus, another god of the underworld. Its surface shows crystalline water ice and probably ammonia, which radiation should slowly destroy, hinting at relatively recent cryovolcanic resurfacing. A large moon, Vanth, was discovered in 2005.
Images
Images from open archives · credit shown per image