  The Kuiper belt object Orcus and its satellite Vanth form an unusual system
in the Kuiper belt. Orcus is amongst the largest objects known in the Kuiper
belt, but the relative size of Vanth is much larger than that of the tiny
satellites of the other large objects. From Hubble Space Telescope observations
we find that Orcus and Vanth have different visible colors and that Vanth does
not share the water ice absorption feature seen in the infrared spectrum of
Orcus. We also find that Vanth has a nearly face-on circular orbit with a
period of 9.5393 +-0.0001 days and semimajor axis of 8980+-20 km, implying a
system mass of 6.32+- 0.01 X 10^20 kg or 3.8% the mass of dwarf planet Eris.
  From Spitzer Space Telescope observations we find that the thermal emission
is consistent with a single body with diameter 940+-70 km and a geometric
albedo of 0.28+-0.04. Assuming equal densities and albedos, this measurements
implies sizes of Orcus and Vanth of 900 and 280 km, respectively, and a mass
ratio of 33. Assuming a factor of 2 lower albedo for the non-icy Vanth,
however, implies sizes of 820 and 640 km and a mass ratio of 2. The measured
density depends on the assumed albedo ratio of the two objects but is
approximately 1.5+-0.3 g cm^-3$, midway between typical densities measured for
larger and for smaller objects. The orbit and mass ratio is consistent with
formation from a giant impact and subsequent outward tidal evolution and even
consistent with the system having now achieved a double synchronous state. The
system can equally well be explained, however, by initial eccentric capture,
Kozai cycling which increases the eccentricity and decreases the pericenter of
the orbit of Vanth, and subsequent tidal evolution inward.
