  We present low resolution K band spectra taken at the Gemini 8 meter
telescope of (90377) Sedna and (90482) Orcus (provisional designations 2003
VB12 and 2004 DW, respectively), currently the two minor planets with the
greatest absolute magnitudes (i.e. the two most reflective minor planets). We
place crude limits on the surface composition of these two bodies using a Hapke
model for a wide variety of assumed albedos. The unusual minor planet (90377)
Sedna was discovered on November 14, 2003 UT at roughly 90 AU with 1.6 times
the heliocentric distance and perihelion distance of any other bound minor
planet. It is the first solar system object discovered between the Kuiper Belt
and the Oort Cloud, and may represent a transition population between the two.
The reflectance spectrum of (90377) Sedna appears largely featureless at the
current signal-to-noise ratio, suggesting a surface likely to be highly
processed by cosmic rays. For large grain models (100 micron to 1 cm) we find
that (90377) Sedna must have less than 70% surface fraction of water ice and
less than 60% surface fraction of methane ice to 3 sigma confidence. Minor
planet (90482) Orcus shows strong water ice absorption corresponding to less
than 50% surface fraction for grain models 25 micron and larger. Orcus cannot
have more than 30% of its surface covered by large (100 mm to 1 cm) methane
grains to 3 sigma confidence.

Astrophysics