Big Exoplanet Science with Small Satellites David R. Ardila1, Varoujan Gorgian1, Mark Swain1, Michael Saing1 Competitive exoplanet science can be done with small telescopes: consortiums like KELT and WASP (transits), MINERVA (radial velocity and transits), OGLE (microlensing), and many others, demonstrate the power of dedicated ground facilities with small apertures to advance the field. Dedicated observations from space with small satellites (SmallSats) would allow dedicated continuous observing, with darker backgrounds, at wavelengths not reachable from the ground. Here we describe some of current and planned SmallSat missions for exoplanet science. The community has produced a number of concept studies that we use to illustrate their potential. With the promise of lower cost, the use of new technology, shorter development times, and frequent access to space, SmallSats provide unique opportunities to advance exoplanet science. What is a Small Satellite? We define a SmallSat as having mass ≤180 kg. The limit comes from the capabilities of the Evolved Expendable Launch Vehicle (EELV) Secondary Payload Adapter (ESPA). Then, another possible definition is a satellite small enough to be a secondary payload. At the upper mass end, a SmallSat will have payload dimensions close to ~50 cm x 50 cm x 50 cm, and will fit a 30-40 cm diameter telescope. At the lower end, the Breakthrough Starshot recently launched six 4 grams satellites (Crane 2017). For comparison, the Galaxy Evolution Explorer (GALEX), had a mass of 277 kg and a telescope diameter of 50 cm (NASA 2003). By this writing, NASA’s Astrophysics Division has never launched a SmallSat, although four CubeSats are planned, two of those for exoplanet science. Despite their absence in astrophysics, SmallSats are common, with the population dominated by CubeSats. Sized in units of 10 cm x 10 cm x 10 cm, each 1U has a mass of 1.3 kg. Over 800 CubeSats, as large as 6U, have been launched. Most SmallSats mentioned here are CubeSats. Exoplanet Science with SmallSats NASA’s exoplanet strategy distinguish between finding exoplanets, characterizing them, and searching for life (NASA 2014). SmallSats can contribute to all these goals. Operationally, they are particularly well-suited for time-domain applications, complement observations from other facilities, and advance technologies to benefit future missions. Finding Exoplanets Direct Imaging: Direct imaging concepts based on coronagraphs and starshades have been proposed for SmallSats, as technology demonstrators and pathfinders for future missions. Centaur (Bendek 2017) is a coronography technology concept consisting of a 15 cm aperture telescope on a 27U bus. The Phase-Induced Amplitude Apodization (PIAA) coronagraph has working angles between 1” and 2.5”, and 10-7 contrast. The PIAA coronagraph is ideal for 1 Jet Propulsion Laboratory/California Institute of Technology. POC: [email protected] systems with small apertures, as it has throughputs ~100%. This performance is comparable to that of the HST/ACS/HRC coronagraph. Centaur is designed to image a Cen A and B and would serve as pathfinder for more ambitious missions. SmallSats can also implement starshades. The miniaturized Distributed Occulter / Telescope system (mDOT – Kolmas et al. 2016, Fig. 1) pairs a 10 cm telescope hosted on a CubeSat with a 2 m diameter starshade at 500 km, hosted on a SmallSat. This design results in a contrast of 10-6 (Kolmas et al. 2016). Wavefront control is needed in coronagraph systems to correct image plane aberrations and speckles due to mirror errors, thermal deformation, and diffraction in the optics. The CubeSat Deformable Mirror Demonstration mission (DeMi) concept detects and corrects wavefront distortion (Douglas et al. 2017). DeMi’s key component is a 140-actuator multi-Deformable Mirror with large stroke (1.5-5.5 µm Figure 1: mDOT project concept (Credit: Space Rendezvous displacement). DeMi’s photometric sensitivity is Laboratory) expected to be 300 ppm. Neither Centaur, mDOT, nor DeMi are intended to compete scientifically or technically with systems like JWST or WFIRST but they would provide crucial experience to concepts like LUVOIR or HabEx. Transit Detection: The transit depth of an Earth-like planet around a Sun-like star is 90 ppm and a 40 cm telescope from space can reach this precision on a V=6 G2 star in 65 sec2. The actual S/N will be limited to smaller values by spacecraft jitter, and errors in the knowledge of the inter/intrapixel gains. Jitter, defined here as high frequency pointing errors, is a serious technical obstacle in SmallSat performance. Flat field errors, paired with the jitter reduce sensitivity. Jitter produces a blur spot, decreasing spatial resolution and increasing read and dark current noise. While HST has a few milliarcseconds jitter, the state of the art for CubeSats is 1s=5”-10” (Sanders et al. 2017). This is a uniquely astrophysical concern, as Earth Sciences satellites use very short exposure times. ASTERIA (the Arcsecond Space Telescope Enabling Research in Astrophysics) provides a possible solution. ASTERIA is a 6U CubeSat hosting a 10 cm telescope and a camera mounted on a piezoelectric stage. During observations of a bright star, the camera is read at a rate of 20 Hz and the focal plane array is moved to compensate for the jitter. Launched from the International Space Station (ISS) on 11/2017, ASTERIA has demonstrated pointing stability of 0.5” over periods on minutes (Knapp 2018). 2 Scaling from HST’s WFC3 performance with the F555W filter. The Paris Observatory-led PicSat mission seeks to solve the same problem using a single pixel avalanche photodiode, also mounted on piezo stage, fed by a 3.5 cm aperture. PicSat is a 3U that was launched on 01/2018 to observe b Pictoris, with the hope of detecting the transit of b Pic b. The system’s predicted sensitivity is under 200 ppm per hour (Nowak et al. 2016). ASTERIA and PicSat will show that transit observations are possible from a SmallSat, opening the door to the full characterization power of the transit technique. Blind planet searches, such as those performed by Kepler, TESS, and PLATO, focusing on specific regions of the sky at a variety of wavelengths are also possible. For example transit searches in different age clusters is a clear application for a dedicated SmallSat (Rizzuto et al. 2017). Other Methods: No SmallSat concepts are publicly available for radial velocity (RV), microlensing, or astrometry. For the first two it would be difficult for a SmallSat to compete with the very stable ground-based precision RV experiments in progress, or with WFIRST. Astrometry from a single SmallSat is probably not possible due to jitter, but it would be possible from multiple spacecraft. Characterizing Exoplanets As mentioned, NASA has approved two exoplanet science CubeSats under the Astrophysics Research and Analysis program (APRA): CUTE (2020, Fleming et al. 2017) and SPARCS (2021, Shkolnik et al. 2018). Both are 6U CubeSats, focused on time-domain observations in the ultraviolet (UV, Figure 2). APRA emphasizes technology innovation and science impact, resulting in ambitious missions at the edge of capability. CUTE’s (the Colorado Ultraviolet Deployable Radiator Transit Experiment) will study Solar panel Deployable exoplanetary mass-loss and magnetic radiator fields in a sample of 12 hot Jupiter UHF transiting systems, over multiple Antenna Antenna Telescope Transceiver transits, expanding on the handful of ACS: star tracker + Patch Antenna 3 axis reaction HST’s UV measurements showing UHF wheel Antenna evidence for star-planet interactions. Paired with ground-based Figure 2: CUTE (left) and SPARCS (right) concepts. See text for references. observations, the mission may provide conclusive evidence of exoplanet magnetic fields. CUTE’s payload is a telescope with a rectangular aperture (20 cm x 8 cm) paired with a R~3700 spectrograph (252 nm — 334 nm), and a “UV-enhanced” CCD. SPARCS (the Star-Planet Activity Research CubeSat) seeks to understand the high-radiation UV environment of planets orbiting M-dwarfs, by observing flare activity and rotational variability in a dozen stars. The targets will be observed for as long as ~3 mo. Knowledge of the episodic emission of M-dwarfs remains poor, specially in the UV, where flares are bright, as the emission maps the upper stellar atmosphere. The physical/chemical origin attributed to the planet’s atmospheric composition requires knowledge of the stellar input spectrum; nowhere is this more true than in reducing the potential for false positives when possible biosignature molecules, such as ozone, also have a potential photochemical origin. SPARCS’ payload is a 8.6 cm aperture mated to a delta-doped CCD with two solar-blind filters (150 nm and 280 nm). The CCD provides ultra-high sensitivity that makes the mission possible with a small aperture. CUTE and SPARCS show that competitive exoplanet science is possible from a SmallSat. Other missions include expanding the exoplanet sample beyond the current limits (mostly late F to early M main sequence stars), understanding planetary system formation histories, characterizing planetary atmospheres, characterizing the hosts, understanding the interaction between star and planets, etc. Spectroscopic, or even narrow-band transit observations can be used to characterize clouds and aerosols in planetary atmospheres, and measure albedo. Dedicated follow-up of individual targets can be used to search for planetary rings, additional planetary members, or biosignatures at different wavelengths. Technological bottlenecks As discussed, jitter is being addressed via moving piezo stages. This solution is appropriate for the transiting science case (bright targets) but not for the characterization of faint sources. Deployable apertures, that would result in larger collecting area, or concepts involving space assembly (e.g. Underwood et al. 2015) have low maturity and are not ready for infusion in a competitive science proposal. Other technology issues in need of maturing include active cooling (for thermal infrared applications), reliability of parts (Venturini 2017), and compact instrument designs.