The Regolith X-ray Imaging Spectrometer (REXIS) will provide an X-ray map of Bennu, complementing core OSIRIS-REx mission science. REXIS brings the strengths of four groups within Massachusetts Institute of Technology (MIT) and Harvard University utilizing the flight proven Conceive, Design, Implement and Operate (CDIO) curriculum, with the potential to involve more than 100 students throughout the process. REXIS is based on flight heritage hardware, thereby minimizing all elements of technical risk, schedule risk, and cost risk.
Sample return is the principal objective of the OSIRIS-REx mission. Maximizing the science yield from the sample analysis requires the best possible asteroid context for the returned sample. The REXIS science objective is to complement onboard mineral mapping by adding spatially resolved elemental abundance mapping achieved through X-ray spectrometry, an objective with heritage proven by the NEAR mission to asteroid Eros.
REXIS is a coded aperture soft X-ray (0.3–7.5 keV) telescope that images X-ray fluorescence line emission produced by the interaction of solar X-rays and the solar wind with the regolith of Bennu. Images are formed with 21-arcminute resolution (4.3 m spatial resolution at a distance of 700 m). Imaging is achieved by correlating the detected X-ray image with a 64 x 64 element random mask (1.536 mm pixels). REXIS will store each X-ray event in order to maximize the data storage usage and to minimize the risk. The pixels will be addressed in 64 x 64 bins and the 0.3–7.5 keV range will be covered by 5 broad bands and 11 narrow line bands. A 24-sec resolution time tag will be interleaved with the event data to account for Bennu rotation. Images will be reconstructed on the ground after downlink of the event list. Images are formed simultaneously in 16 energy bands centered on the dominant lines of abundant surface elements from O-K (0.5 keV) to Fe-Kß (7 keV) as well the representative continuum. During orbital phase 5B, a 21-day orbit 700 m from the surface of Bennu, a total of at least 133 events/asteroid pixel/energy band are expected under 2 keV; enough to obtain significant constraints on element abundances at scales larger than 10 m.
REXIS has been selected as an education and public outreach element of the OSIRIS-REx mission investigation after a competitive selection process. As a Student Collaboration Experiment (SCE), REXIS will be incorporated into the OSIRIS-REx spacecraft on a non-impact basis and will remain clearly separable from the rest of the OSIRIS-REx investigation. Data analysis will be handled as a student project, available to both Harvard and MIT students, for credit or non-credit within the MIT curriculum, and within the MIT Undergraduate Research Opportunities Program (UROP). Over 15 semesters, more than 100 undergraduate, and more than 10 graduate students from MIT and Harvard are expected to participate in the REXIS project.
REXIS is a collaboration between students and faculty at MIT and Harvard University. At MIT, faculty leadership is provided by Professor David Miller, Professor Richard Binzel, and Professor Sara Seager. At Harvard, faculty leadership is provided by Professor Josh Grindlay.