Guest Bloggers: Mark Chodas (REXIS Student Team Members) and Richard P. Binzel (REXIS-PI)
When the OSIRIS-REx spacecraft arrives at the target near-Earth asteroid Bennu, its first science task is to globally characterize the nature and composition of the asteroid surface. This characterization is all driven toward the selection among safe and enticing sampling sites for the primary mission goal of returning a sample to Earth.
OSIRIS-REx carries a suite of cameras that will image Bennu’s surface and measure its colors and composition in a way that is “familiar” for most planetary exploration missions. However, one instrument will measure Bennu in a very different light and it is also coming aboard OSIRIS-REx in a very special and exciting way. The Regolith X-ray Imaging Spectrometer (or REXIS) is being created as a hands-on student-built project at the Massachusetts Institute of Technology in collaboration with the Harvard College Observatory. REXIS will “see” the surface of Bennu in X-ray light, giving a distinctly different view of the asteroid. That view won’t let us look “all the way through” the asteroid in the way we think of a medical X-ray, but it will let us look just enough inside the rocks and soil (which geologists call “regolith”) on the surface to tell us about the atomic elements that comprise them. These measurements of atomic elements form a perfect complement to other OSIRIS-REx payload instruments that will determine the mineral chemistry of the surface. Combining the atomic element view with the mineral view the will give us the most accurate depiction of what Bennu is made of that can be achieved from orbit. We will use that information to guide site selection to give the best possible context for the composition and solar system history that will later be revealed by the full capability of Earth-based laboratories analyzing the returned sample.
Like every other instrument aboard OSIRIS-REx, REXIS requires the careful design and testing of many components both individually and as an integrated package. REXIS is comprised of two subassemblies: the Spectrometer and the Solar X-ray Monitor (SXM). The Spectrometer observes the asteroid while the SXM observes the Sun. Because the Sun’s X-ray output affects Bennu’s X-ray output, we need to keep track of what the Sun is doing, including solar flares, in order to calibrate our Bennu data correctly. The Spectrometer collects X-ray photons from Bennu using four charge coupled devices (CCDs) but before the photons are detected by the CCDs, they pass through a coded aperture mask. The mask is a random pattern of open and closed holes in a thin stainless steel sheet. By analyzing how the shadow of the mask pattern is shifted on the CCDs, we can determine areas of high X-ray activity on the asteroid. This is how REXIS takes “images” of Bennu without any mirrors or lenses like the other instruments on OSIRIS-REx. Because the CCDs are damaged by the radiation environment in deep space, an aluminum cover over the coded aperture mask blocks charged particles from reaching the CCDs during the cruise phase. The cover will open just before REXIS starts observations of the asteroid. The Spectrometer also holds the electronics that make up the “brains” of REXIS. To improve the energy resolution of the CCDs, that is how well the detectors can measure the energy of a photon, they must be cooled to below -60 C. Therefore, REXIS has a thermal strap and radiator to cool the CCDs and thermal isolation between the warm electronics box and the cold detectors. The Spectrometer is mounted on the asteroid-facing instrument deck of OSIRIS-REx with the other instruments but, in order to observe the Sun, the SXM is mounted on the Sun-facing side of the spacecraft. A cable connects the SXM to the electronics box of the spectrometer.
The fundamental purpose of REXIS is to educate the next generation of scientists and engineers by engaging students in every facet of development. REXIS is designed, built, tested, and operated by a student team at MIT, assisted by mentors at MIT and Harvard. So far, over 40 undergraduate and 12 graduate students have worked on REXIS with more to come as REXIS gets closer to launch. Opportunities for students to work on flight hardware are rare yet incredibly valuable. The realities of developing space hardware and software can’t just be taught in a classroom. The students who work on REXIS will enter the workforce with unique skills and experiences that will be a benefit to the aerospace industry.
What does REXIS hope to accomplish? Our objective is to measure the abundances of specific atomic elements that are particularly active in response to being struck by solar X-rays. For the sensitivity range of our CCDs, we will be able to measure how strongly Bennu “glows” in the light generated by the elements magnesium (Mg), iron (Fe), sulfur (S), and silicon (Si). These are of keen scientific interest because these are the same elements present in most meteorite groups, giving OSIRIS-REx an early look at what Bennu is made of, heightening the anticipation of what powerful Earth-based laboratories will be able analyze with the observed sample. If REXIS determines that the abundances of these elements vary from region to another on the asteroid, we will add this extra information to the project team as a bonus for choosing a sample site. Each step along the way, the student team will be there learning first hand the methods and challenges of modern spacecraft missions. All students involved in whole or in part from start to finish, will carry forward into their careers a sense of accomplishment of working at the frontier of planetary exploration.