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Big 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 Division has never launched a SmallSat, although four 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 , 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: David.R.Ardila@jpl..gov 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 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 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 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 . 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 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 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 -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 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 . 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.

Technology development for astrophysics SmallSats remains problematic. The Small

Spacecraft Technology program, part of the Space Technology Mission Directorate (STMD) has emphasized bus capabilities (communication, proximity operations, propulsion) which are not particularly relevant to exoplanet science or astrophysics. Within astrophysics, the main opportunity to validate new SmallSat technology in space is APRA, via sounding rockets, and very

Figure 3. SmallSat buses, high mass end. Clockwise from the top limited CubeSat opportunities. On the other hand, left: BCT-100, AMA MAGICBus, XB Microsat, Harris’ Earth Sciences’ In-Space Validation of Earth SpaceView. Science Technologies (InVEST) program provides the model for a successful program. While not geared specifically towards SmallSats, the InVest program has resulted in a large number of CubeSats missions, developing a wide array of ambitious instruments in compact packages, including thermal infrared spectrographs and active radar.

Vendors There is a robust ecosystem of vendors and services around CubeSats, and vendors for larger SmallSats are also available. For example, Ball Aerospace’s BCP-100, Adcole Maryland Aerospace MAGICBus, Harris’ Spaceview series, and Blue Canyon’s XB Microsat will fit an ESPA ring and accommodate payloads with 30% - 40% the total spacecraft mass, and telescopes ~40 cm in aperture (Figure 3). In what may be a typical example, Surrey’s NASA Rapid III SSTL-150 bus is advertised at $18 M.3

Programmatic landscape Table 1. NASA Astrophysics calls allowing Table 1 shows Astrophysics funding opportunities. For a SmallSats SmallSats NASA satellite to LEO, a good rule of thumb is that the AO Cost Cap funded cost is equally split between payload, bus, reserves, and APRA ~$5M 4 CubeSats everything else. For a 180 kg satellite the lifecycle cost $175M (FY15, SMEX None inc. launch) will be between $70M to $100M depending on the MO – Suborbital $35M (FY17) None complexity of the instrument and the expertise of the (<6U Cubesats) 4 MO – Small team . University-developed satellites will encounter $70M (F17, inc. Complete None launch) lower costs, although how much lower is difficult to tell. Mission Given this, it is surprising that more astrophysics SmallSats have not been launched. It seems that the community is not aware of the opportunities available. A common argument against SmallSats is that their capabilities are limited, compared to Explorers, Probes, or Flagships. From this tautological argument one concludes that NASA should only fund Flagships, which by virtue of being the expensive missions should be also the most capable. In reality, the astronomical community has consistently supported the value of a diverse portfolio of missions, at many cost points, and with complementary capabilities. Larger missions need to be shared among an array of scientific communities, and specific science activities requiring large amounts of time, specialized instruments, or fast development, are simply not possible.

Conclusions and Suggestions This WP is intended to be a conversation with the community, as we participate in the Decadal process, and in that spirit we offer the following suggestions: • Create a bi-annual AO call for astrophysics SmallSat technology with a flight demonstration. This would be modeled like the InVest AO. • Increase the APRA Astrophysics budget for CubeSats. As structured now, funding is the limiting factor in the development of exoplanet science CubeSats. Based on multiple experiences, the cost of a competitive exoplanet mission is closer to $10M than $5M. • Explore ways to reduce SmallSat cost: for example, provide a standard set of SmallSat buses, and negotiate their prices with vendors.

References: Bendek, E. 2017, APRA Fact Sheet, goo.gl/aXc7w7, Retr. 2/25/2018 • Crane, L., New Sci., 26 07 2017• Douglas, E. S. et al. 2017, Proc. SPIE, 10400, 13-1 • Fleming, B. T. et al. 2017, Proc. SPIE, 10397,1• Knapp, M. 2018, pers. comm. • Kolmas, J. et al. 2016, 2016 IEEE Aero. Conf., 1-11. • NASA Press Kit: GALEX Launch, 4/2003. • NASA 2014, Science Plan • Nowak, M. et al. 2016, Proc. SPIE, 9904, 4 • Rizzuto et al. 2017, arXiv:1709.09670v2 • Sanders, D. et al., Small Satellite Conference Proceedings, 2017. • Shkolnik, E. et al., 2018, AAS 231, 228.04 • Underwood, C. et al. 2015. Acta Astronautica 114, 112. • Venturini, C.C. 2017, Aerospace Report No. TOR-2017-01689 • Wertz, James Richard, et al. Space mission engineering: the new SMAD. Microcosm Press, 2015.

3 https://www.sst-us.com/shop/satellite-platforms/nasa-rapid-iii-sstl-150-50kg-50w 4 NASA’s recent request for information on Astrophysics SmallSats had a cost cap of $35M, which results in missions <180 kg.