ASTERIA (Arcsecond Space Telescope Enabling Research In Astrophysics) is a 6U CubeSat space telescope (10 cm x 20 cm x 30 cm, 10 kg). ASTERIA's primary mission objective was demonstrating two key technologies for reducing systematic noise in photometric observations: high-precision pointing control and high-stabilty thermal control. ASTERIA demonstrated 0.5 arcsecond RMS pointing stability and ±10 milliKelvin thermal control of its camera payload during its primary mission, a significant improvement in pointing and thermal performance compared to other spacecraft in ASTERIA's size and mass class. ASTERIA launched in August 2017 and deployed from the International Space Station (ISS) November 2017. During the prime mission (November 2017 -February 2018) and the first extended mission that followed (March 2018 -May 2018), ASTERIA conducted opportunistic science observations which included collection of photometric data on 55 Cancri, a nearby exoplanetary system with a super-Earth transiting planet. The 55 Cancri data were reduced using a custom pipeline to correct CMOS detector column-dependent gain variations. A Markov Chain Monte Carlo (MCMC) approach was used to simultaneously detrend the photometry using a simple baseline model and fit a transit model. ASTERIA made a marginal detection of the known transiting exoplanet 55 Cancri e (∼ 2 R ⊕ ), measuring a transit depth of 374 ± 170 ppm. This is the first detection of an exo-
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.
A guidance and a control law are developed to maneuver the boresight axis of a spacecraft relative to a reference vector, using knowledge of the reference vector and the angular rates of the spacecraft. Therefore, this can be used in cases when the full attitude is not known or when the angle of the spacecraft around the boresight or reference vector does not need to be controlled. A Lyapunov-based analysis is performed to prove asymptotic stability of the system to a reference trajectory. Even in the presence of inertia uncertainty, it is shown that asymptotic stability to a stationary attitude can be guaranteed. To assist with practical gain selection, a linear analysis is performed. Finally, the boresight guidance and control laws are applied to four different scenarios to demonstrate their utility. The first two show examples of pointing in an inertially-fixed direction and tracking a time-varying target. The third and fourth demonstrate searching tasks. A novel example of searching for the Sun by reducing the search space from a full 4πsteradian sphere to a circle of a sphere is presented. If the full sphere must be searched, a spherical spiral maneuver is presented to perform this task.
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