  OSIRIS-REx is the third spacecraft in the NASA New Frontiers Program and is
planned for launch in 2016. OSIRIS-REx will orbit the near-Earth asteroid
(101955) Bennu, characterize it, and return a sample of the asteroid's regolith
back to Earth. The Regolith X-ray Imaging Spectrometer (REXIS) is an instrument
on OSIRIS-REx designed and built by students at MIT and Harvard. The purpose of
REXIS is to collect and image sun-induced fluorescent X-rays emitted by Bennu,
thereby providing spectroscopic information related to the elemental makeup of
the asteroid regolith and the distribution of features over its surface.
Telescopic reflectance spectra suggest a CI or CM chondrite analog meteorite
class for Bennu, where this primitive nature strongly motivates its study. A
number of factors, however, will influence the generation, measurement, and
interpretation of the X-ray spectra measured by REXIS. These include: the
compositional nature and heterogeneity of Bennu, the time-variable Solar state,
X-ray detector characteristics, and geometric parameters for the observations.
In this paper, we will explore how these variables influence the precision to
which REXIS can measure Bennu's surface composition. By modeling the
aforementioned factors, we place bounds on the expected performance of REXIS
and its ability to ultimately place Bennu in an analog meteorite class.
