  The OSIRIS-REx Mission was selected under the NASA New Frontiers program and
is scheduled for launch in September of 2016 for a rendezvous with, and
collection of a sample from the surface of asteroid Bennu in 2019. 101955 Bennu
(previously 1999 RQ36) is an Apollo (near-Earth) asteroid originally discovered
by the LINEAR project in 1999 which has since been classified as a potentially
hazardous near-Earth object. The REgolith X-Ray Imaging Spectrometer (REXIS)
was proposed jointly by MIT and Harvard and was subsequently accepted as a
student led instrument for the determination of the elemental composition of
the asteroid's surface as well as the surface distribution of select elements
through solar induced X-ray fluorescence. REXIS consists of a detector plane
that contains 4 X-ray CCDs integrated into a wide field coded aperture
telescope with a focal length of 20 cm for the detection of regions with
enhanced abundance in key elements at 50 m scales. Elemental surface
distributions of approximately 50-200 m scales can be detected using the
instrument as a simple collimator. An overview of the observation strategy of
the REXIS instrument and expected performance are presented here.
