90482 Orcus - Orbit and Rotation

Orbit and Rotation

Orcus is in 2:3 orbital resonance with Neptune, having an orbital period of 247 years. Orcus's orbit is similar to Pluto's (both have perihelia above the ecliptic), but is oriented differently. Although at one point its orbit approaches that of Neptune, the resonance between the two bodies means that Orcus itself is always a great distance away from Neptune (there is always an angular separation of over 60 degrees between them). Over a 14,000-year period Orcus stays more than 18 AU from Neptune. Because their mutual resonance with Neptune constrains Orcus and Pluto to remain in opposite phases of their otherwise very similar motions, Orcus is sometimes described as the "anti-Pluto".

Orcus is currently 47.8 AU from the Sun and will come to aphelion (farthest distance from the Sun) in 2019. Simulations by the Deep Ecliptic Survey (DES) show that over the next 10 million years Orcus can acquire a perihelion distance (qmin) as small as 27.8 AU.

The rotation period of the primary is not known. Different photometric surveys have produced different results. Some show low amplitude variations with periods ranging from 7 to 21 hours, while others show no variability. However the value obtained by Ortiz et al., about 10.5 hours seems to be the most likely. The rotational poles of Orcus probably coincide with the orbital poles of its moon, Vanth. This means that Orcus is currently viewed pole-on, which could explain the near absence of any rotational modulation of its brightness. If, however, the primary is tidally locked with the satellite, the rotational period will coincide with the 9.7-day orbital period of Vanth.