OSIRIS-REx is a NASA New Frontiers mission scheduled for launch in 2016 that will travel to the asteroid Bennu and return a pristine sample of the asteroid to Earth. The REgolith X-ray Imaging Spectrometer (REXIS) is a student collaboration instrument on-board the OSIRIS-REx spacecraft. REXIS is a NASA risk Class D instrument, and its design and development is largely student led. The engineering team consists of MIT graduate and undergraduate students and staff at the MIT Space Systems Laboratory. The primary goal of REXIS is the education of science and engineering students through participation in the development of light hardware. In light, REXIS will contribute to the mission by providing an elemental abundance map of the asteroid and by characterizing Bennu among the known meteorite groups. REXIS is sensitive to X-rays between 0.5 and 7 keV, and uses coded aperture imaging to map the distribution of iron with 50 m spatial resolution. This paper describes the science goals, concept of operations, and overall engineering design of the REXIS instrument. Each subsystem of the instrument is addressed with a high-level description of the design. Critical design elements such as the Thermal Isolation Layer (TIL), radiation cover, coded-aperture mask, and Detector Assembly Mount (DAM) are discussed in further detail.
CubeSats are a class of nanosatellites that conform to a standardized 10 cm x 10 cm x 10 cm, 1 kg form factor.
This miniaturization, along with a standardized deployment device for launch vehicles, allows CubeSats to be
launched at low cost by sharing the trip to orbit with other spacecraft. Part of the original motivation for the
CubeSat platform was also to allow university students to participate more easily in space technology development
and to gain hands-on experience with flight hardware. The Department of Aeronautics and Astronautics along
with the Department of Earth, Atmospheric, and Planetary Studies (EAPS) at the Massachusetts Institute of
Technology (MIT) recently completed a three semester-long course that uses the development of a CubeSat-based
science mission as its core teaching method. Serving as the capstone academic experience for undergraduates,
the goal of this class is to design and build a CubeSat spacecraft that serves a relevant science function, such
as the detection of exoplanets transiting nearby stars. This project-based approach gives students essential
first hand insights into the challenges of balancing science requirements and engineering design. Students are
organized into subsystem-specific teams that refine and negotiate requirements, explore the design trade space,
perform modeling and simulation, manage interfaces, test subsystems, and finally integrate prototypes and flight
hardware. In this work we outline the heritage of capstone design/build classes at MIT, describe the class format
in greater detail, and give results on the ability to meet learning objectives using this pedagogical approach.
KEYWORDS: Stars, Space operations, Charge-coupled devices, Exoplanets, Interference (communication), Sensors, Planets, Signal to noise ratio, Space telescopes, CCD image sensors
Nanosatellites, i.e. spacecraft that weigh between 1 and 10 kg, are drawing increasing interest as platforms
for conducting on-orbit science. This trend is primarily driven by the ability to piggyback nanosatellites on
the launch of large spacecraft and hence achieve orbit at greatly reduced cost. The CubeSat platform is a
standardized nanosatellite configuration, consisting of one, two, or three 10 cm x 10 cm x 10 cm units (1, 2,
or 3 "U"s) arranged in a row. We present a CubeSat-based concept for the discovery of transiting exoplanets
around the nearest and brightest Sun-like stars. The spacecraft prototype - termed ExoplanetSat - is a 3U space
telescope capable of monitoring a single target star from low Earth orbit. Given the volume limitations of
the CubeSat form factor, designing a capable spacecraft requires overcoming significant challenges. This work
presents the initial satellite configuration along with several subsystem-specific solutions to the aforementioned
constraints. An optical design based on a modified commercial off-the-shelf camera lens is given. We also
describe a novel two-stage attitude control architecture that combines 3-axis reaction wheels for coarse pointing
with a piezoelectric translation stage at the focal plane for fine pointing. Modeling and simulation results are
used to demonstrate feasibility by quantifying ExoplanetSat pointing precision, signal-to-noise ratio, guide star
magnitude, and additional design parameters which determine system performance.
ExoplanetSat is a proposed three-unit CubeSat designed to detect down to Earth-sized exoplanets in an orbit
out to the habitable zone of Sun-like stars via the transit method. To achieve the required photometric precision
to make these measurements, the target star must remain within the same fraction of a pixel, which is equivalent
to controlling the pointing of the satellite to the arcsecond level. The satellite will use a two-stage control
system: coarse control will be performed by a set of reaction wheels, desaturated by magnetic torque coils, and
fine control will be performed by a piezoelectric translation stage. Since no satellite of this size has previously
demonstrated this high level of pointing precision, a simulation has been developed to prove the feasibility of
realizing such a system.
The current baseline simulation has demonstrated the ability to hold the target star to within 0.05 pixels
or 1.8 arcseconds (with an 85 mm lens and 15 μm pixels), in the presence of large reaction wheel disturbances
as well as external environmental disturbances. This meets the current requirement of holding the target star
to 0.14 pixels or 5.0 arcseconds. Other high-risk aspects of the design have been analyzed such as the effect of
changing the guide star centroiding error, changing the CMOS sampling frequency, and reaction wheel selection
on the slew performance of the satellite. While these results are promising as an initial feasibility analysis,
further model improvements and hardware-in-the-loop tests are currently underway.
KEYWORDS: Actuators, Mirrors, Space telescopes, Silicon carbide, Manufacturing, Finite element methods, Space mirrors, Spatial frequencies, Thermal modeling, Disk lasers
The trend in future space telescopes points toward increased primary mirror diameter, which improves resolution
and sensitivity. However, given the constraints on mass and volume deliverable to orbit by current launch
vehicles, creative design solutions are needed to enable increased mirror size while keeping mass and volume
within acceptable limits. Lightweight, segmented, rib-stiffened, actively controlled primary mirrors have emerged
as a potential solution. Embedded surface-parallel actuators can be used to change the mirror prescription onorbit,
lowering mirror mass overall by enabling lighter substrate materials such as silicon carbide (SiC) and
relaxing manufacturing constraints. However, the discrete nature of the actuators causes high spatial frequency
residual errors when commanding low-order prescription changes. A parameterized finite element model is used
to simulate actuator-induced residual error and investigate design solutions that mitigate this error source.
Judicious specification of mirror substrate geometry and actuator length is shown to reduce actuator-induced
residual while keeping areal density constant. Specifically, a sinusoidally-varying rib shaping function is found to
increase actuator influence functions and decrease residual. Likewise, longer actuators are found to offer reduced
residual. Other options for geometric shaping are discussed, such as rib-to-facesheet blending and the use of two
dimensional patch actuators.
We present a high-resolution soft x-ray grating spectrometer concept for the International X-Ray Observatory
(IXO) that meets or exceeds the minimum requirements for effective area (> 1, 000 cm2 for E < 1 keV) and
spectral resolution (E/▵E > 3, 000). At the heart of the spectrometer is an array of recently developed highefficiency
blazed transmission gratings, the so-called critical-angle transmission (CAT) gratings. They combine
the advantages of traditional transmission gratings (very low mass, extremely relaxed alignment and flatness tolerances)
with those of x-ray reflection gratings (high efficiency due to blazing in the direction of grazing-incidence
reflection). In addition, a CAT grating spectrometer is well-suited for co-existence with energy-dispersive highenergy
focal plane detectors, since most high-energy x rays are neither absorbed, nor diffracted, and contribute
to the effective area at the telescope focus. Since our initial successful x-ray demonstrations of the CAT grating
concept with large-period and lower aspect-ratio prototypes, we have now microfabricated 200 nm-period silicon
CAT gratings comprised of grating bars with the required dimensions (6 micron tall, 40 nm wide, aspect ratio
150), optimized for the 0.3 to 1.0 keV energy band. Preliminary analysis of recent x-ray tests show blazing
behavior up to 1.28 keV in accordance with predictions.
The trend in future space telescopes is towards larger apertures, which provide increased sensitivity and improved
angular resolution. Lightweight, segmented, rib-stiffened, actively controlled primary mirrors are an enabling
technology, permitting large aperture telescopes to meet the mass and volume restrictions imposed by launch
vehicles. Such mirrors, however, are limited in the extent to which their discrete surface-parallel electrostrictive
actuators can command global prescription changes. Inevitably some amount of high spatial frequency residual
error is added to the wavefront due to the discrete nature of the actuators. A parameterized finite element
mirror model is used to simulate this phenomenon and determine designs that mitigate high spatial frequency
residual errors in the mirror surface figure. Two predominant residual components are considered: dimpling
induced by embedded actuators and print-through induced by facesheet polishing. A gradient descent algorithm
is combined with the parameterized mirror model to allow rapid trade space navigation and optimization of the
mirror design, yielding advanced design heuristics formulated in terms of minimum machinable rib thickness.
These relationships produce mirrors that satisfy manufacturing constraints and minimize uncorrectable high
spatial frequency error.
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