CUSP: CubeSat Solar Polarimeter

• CUSP is a CubeSat mission featuring a dual-phase Compton polarimeter that measures linear polarization in the 25–100 keV range to probe magnetic reconnection and particle acceleration.
• The mission utilizes time-resolved measurements and rigorous calibration methods to minimize systematic errors and enhance sensitivity in probing solar flare dynamics.
• Developed under the ASI Alcor Program, CUSP integrates advanced electronics, precise attitude control, and environmental testing to validate CubeSat instrumentation for heliophysics.

CUSP, the CUbesat Solar Polarimeter, is an Earth-orbiting CubeSat mission designed to measure the linear polarization of solar flares in the hard X-ray band with a Compton scattering polarimeter (Angelis et al., 16 Sep 2025). Its core science case is to constrain magnetic reconnection and particle acceleration in flaring magnetic structures by exploiting the polarization dependence of Compton scattering in the 25–100 keV range. Developed within the Alcor Program of the Italian Space Agency, CUSP entered a 12-month Phase B in December 2024 and, in the 2025 mission papers, is presented as a baseline single-satellite implementation derived from an earlier two-CubeSat concept (Fabiani et al., 1 Aug 2025).

1. Scientific rationale

The primary objective of CUSP is to measure the linear polarization of hard X-rays from solar flares and thereby probe the physical conditions under which flare photons are produced (Fabiani et al., 1 Aug 2025). In the mission literature, polarization is treated as a diagnostic of electron acceleration, beaming, pitch-angle distributions, and magnetic field geometry, with particular emphasis on distinguishing thermal from non-thermal emission components. The scientific motivation explicitly includes unresolved questions concerning the geometry of reconnection sites, particle acceleration efficiency, and the relation between solar flares and coronal mass ejections, with direct implications for space weather forecasting (Angelis et al., 16 Sep 2025).

The observational strategy is oriented toward time-resolved measurements of polarization degree and polarization angle on timescales ranging from minutes to hours (Fabiani et al., 1 Aug 2025). This temporal dimension is central because flare evolution can alter both the anisotropy of the emitting electron population and the topology of the magnetic structures involved. The mission papers present CUSP as addressing the long-standing limitation that previous hard X-ray solar polarimetry measurements have been statistically limited and insufficient to distinguish clearly among competing flare models (Fabiani et al., 1 Aug 2025).

A secondary scientific capability follows from the instrument field of view: the mission overview notes possible serendipitous observations of other hard X-ray sources, including the Crab Nebula, Sco X-1, and gamma-ray bursts during solar observations (Fabiani et al., 1 Aug 2025). This suggests that, although the mission is optimized for solar flare polarimetry, the payload may also contribute opportunistically to high-energy transient studies.

2. Mission architecture and programmatic evolution

In its 2025 baseline, CUSP is a 6U XL CubeSat without onboard propulsion, with 2.5U reserved for the polarimeter, a nominal lifetime of 3 years in Low Earth Orbit, and a mission design centered on continuous solar monitoring through sun-pointing attitude control (Fabiani et al., 1 Aug 2025). The platform is specified with ~30 W maximum power using deployable solar panels, an 84 Wh battery, and S-band downlink for science data with up to 10 Mbps max, 5 Mbps nominal, while UHF is used for telemetry and commands (Fabiani et al., 1 Aug 2025). The principal ground-segment elements are a Primary Ground Station and Mission Operations Center at the University of “La Tuscia,” Viterbo, and a possible Science Operations Center at the ASI Space Science Data Center in Rome (Fabiani et al., 1 Aug 2025).

The orbit analysis in the mission overview identifies a Sun-Synchronous Orbit at approximately 500–525 km altitude as the reference solution (Fabiani et al., 1 Aug 2025). For science operations, the spacecraft is intended to remain Sun-pointed, with rotation around the boresight at $\geq 1$ RPM during observations in order to minimize spurious modulation, while 3-axis stabilization is used for data downlink (Fabiani et al., 1 Aug 2025). The mission papers quote <2° attitude precision during science mode and <0.04° @ 3$\sigma$ absolute knowledge error in the technical specification set (Fabiani et al., 1 Aug 2025).

A notable design evolution concerns the constellation concept. Early publications described CUSP as a constellation of two CubeSats in the same orbital plane and 180° phase difference, motivated by improved solar visibility and continuous monitoring (Fabiani et al., 2022). The Phase B mission overview, however, states that the two-satellite constellation was considered and then discarded after trade-off because of rapid loss of phase due to atmospheric drag and orbital uncertainties, leading to a single-satellite mission as the baseline (Fabiani et al., 1 Aug 2025). The same analysis reports that a two-satellite configuration would have improved flare coverage and improved MDP by about 1.4x, but the retained baseline is a single platform with Sun observation duty cycle of about 69% in a dawn-dusk SSO and about 45% in noon-midnight or mid-morning SSO (Fabiani et al., 1 Aug 2025).

Programmatically, CUSP is developed within the ASI Alcor Program, with INAF-IAPS identified as Prime Contractor and PI institution, and contributions from INAF-OAS Bologna, INAF-OAR, Deda Connect s.r.l., IMT s.r.l., the University of Bologna, and the University of Viterbo (Fabiani et al., 1 Aug 2025). The mission papers report that Phase B spans December 2024–December 2025, with an explicit objective of advancing payload and subsystem maturity from TRL 3 to 4 (Fabiani et al., 1 Aug 2025).

3. Payload and detector architecture

CUSP carries a dual-phase Compton scattering polarimeter composed of a low-$Z$ scattering stage and a high-$Z$ absorption stage (Angelis et al., 16 Sep 2025). The scatterer section consists of 64 plastic scintillator bars arranged in an $8\times8$ array, read out by 4 Multi-Anode PhotoMultiplier Tubes, while the absorber stage contains 32 GAGG:Ce crystals organized in 4 strips of 8 crystals surrounding the scatterer array and read out by Avalanche PhotoDiodes (Angelis et al., 16 Sep 2025). In the prototype-development paper, the relevant hardware is specified more concretely as Hamamatsu R7600-03-M16 MAPMTs for the scatterers and Hamamatsu S8664-55 APDs for the absorbers, with MAROC-3A and SKIROC-2A ASICs as the front-end electronics for the two readout chains (Angelis et al., 1 Aug 2025).

The polarimeter geometry is designed so that an incoming photon first undergoes a Compton scatter in the plastic scintillator and is then absorbed in a GAGG crystal (Angelis et al., 16 Sep 2025). CUSP therefore selects time-coincident events between the two acquisition chains as its basic polarimetric observable. Because the first interaction site is identified in the plastic array and the second in the GAGG array, the instrument can reconstruct the azimuthal scattering angle for each accepted event, which is the quantity statistically linked to the source polarization (Angelis et al., 16 Sep 2025).

Passive collimation and shielding are integral parts of the payload design. The instrument uses a tungsten collimator that restricts the field of view to approximately $\pm 36^\circ$ around the solar direction in the Phase B design (Fabiani et al., 1 Aug 2025). The multi-physics design papers further describe the payload as incorporating top and bottom enclosures, side panels with passive tungsten shielding, an optical frame for detector alignment, and a collimator tray that holds the collimators and filters (Lombardi et al., 4 Aug 2025). This configuration reflects a design priority in which background suppression, structural survivability, and optical alignment are treated jointly rather than as separable subsystems.

4. Polarimetric formalism and event reconstruction

CUSP exploits the polarization dependence of the Klein–Nishina differential Compton cross-section. In the mission overview, the relevant expression is written as

$$\frac{d\sigma}{d\Omega} = \frac{r_0^2}{2}\frac{E'^2}{E^2}\left[\frac{E}{E'}+\frac{E'}{E}-2\sin^2\theta\cos^2\phi\right],$$

with

$$\frac{E'}{E}=\frac{1}{1+\frac{E}{m_ec^2}(1-\cos\theta)}.$$

Here $E$ and $E'$ are the incident and scattered photon energies, $\sigma$0 is the polar scattering angle, and $\sigma$1 is the azimuthal angle measured relative to the polarization vector (Fabiani et al., 1 Aug 2025). The physical implication stated throughout the CUSP papers is that linearly polarized photons scatter preferentially perpendicular to the electric field vector, producing a sinusoidal modulation in the azimuthal-angle distribution.

Operationally, CUSP reconstructs a modulation curve from many coincident scatterer–absorber events. The standard sensitivity descriptors are the modulation factor $\sigma$2, the quality factor $\sigma$3, and the Minimum Detectable Polarization (MDP). The mission overview defines

$\sigma$4

and

$\sigma$5

where $\sigma$6 is the detector efficiency (Fabiani et al., 1 Aug 2025). For statistical sensitivity at 99% confidence, the same paper gives

$\sigma$7

with $\sigma$8 the source count rate, $\sigma$9 the background count rate, and $Z$0 the observing time (Fabiani et al., 1 Aug 2025).

The Geant4 performance paper adds a Stokes-parameter formulation for event-by-event analysis. For event $Z$1, it defines

$Z$2

with normalized sums

$Z$3

and modulation amplitude

$Z$4

For a fully polarized beam, $Z$5, while the inferred polarization degree and angle are obtained from

$Z$6

This same study emphasizes that the square geometry of the detector introduces spurious modulation even for unpolarized beams, so the raw azimuthal distribution must be corrected before astrophysical interpretation (Kumar et al., 1 Aug 2025).

5. Simulated and laboratory performance

The scientific-performance evaluation relies on a detailed CUSP Mass Model in Geant4, generated by converting the mechanical CAD geometry into GDML so that both active and passive structures can be simulated consistently (Kumar et al., 1 Aug 2025). The simulation campaign is used to assess the effective area, the modulation factor, the impact of passive materials, and the systematics associated with geometry-driven anisotropies. The 2024 simulation paper describes event classes such as 1 scatterer + 1 absorber, 2 scatterers + 1 absorber, and related coincidence families, and identifies single-scatterer plus absorber coincidences as the main channel for polarimetric events (Cesare et al., 2024).

A central result of the 2025 Geant4 optimization paper is that unpolarized beams produce a four-peak structure in the azimuthal distribution after position randomization because of the payload’s square geometry (Kumar et al., 1 Aug 2025). The same work reports two correction strategies, M1 and M2, based on Stokes-parameter arithmetic, and states that after correction the residual spurious modulation is below 1% (second harmonic), even without dedicated backscattering shielding (Kumar et al., 1 Aug 2025). It also evaluates molybdenum layers placed above and below the plastic scintillator array: these improve the modulation factor by ~1.7% but reduce the effective area by ~5%, making the shielding problem an explicit sensitivity trade-off (Kumar et al., 1 Aug 2025).

The expected polarimetric sensitivity reported in the mission-overview literature is summarized below for the 25–100 keV band at 99% confidence (Fabiani et al., 1 Aug 2025).

Flare class	Integration time (s)	MDP (%)
M 5.2	284	7.8
X 1.2	240	3.9
X 10	351	0.9

These estimates were derived from real solar-flare data and are meant to characterize performance on minute-scale integrations (Fabiani et al., 1 Aug 2025). The same literature states that Geant4 Monte Carlo simulations, together with laboratory tests on development boards and prototypes, show close agreement between expected and prototype performance (Angelis et al., 16 Sep 2025).

Prototype calibration has proceeded in parallel with the simulation effort. The scatterer chain has been tested with an EJ-204 plastic bar coupled to a MAPMT and read out by a MAROC-3A development board, while the absorber chain has used a GAGG bar coupled to an APD and read out by a SKIROC-2A development board (Angelis et al., 1 Aug 2025). The reported radioactive sources include $Z$7Fe, $Z$8Am, and $Z$9Cd for the scatterer calibration, and $Z$0Am, $Z$1Cd, $Z$2Co, $Z$3Eu, and $Z$4I for the absorber calibration (Angelis et al., 1 Aug 2025). The prototype paper states that both acquisition chains show linear response across the required energy interval, with absorber measurements demonstrating consistent signal and good linearity across 25–100 keV, and that a full representative prototype was in construction during Phase B (Angelis et al., 1 Aug 2025).

6. Engineering qualification, mission status, and significance

CUSP’s development has included a substantial multi-physics analysis program covering structural, vibrational, and thermo-mechanical behavior (Lombardi et al., 4 Aug 2025). The engineering workflow uses SolidWorks for full parametric CAD, ANSYS SpaceClaim for defeaturing, ANSYS Meshing for grid generation, and ANSYS Workbench for multiphysics simulation management (Lombardi et al., 4 Aug 2025). The principal analyses include quasi-static loads of 20g, modal analysis, and random vibration analysis using ECSS, GEVS, and NASA standards over the 20–2000 Hz band (Lombardi et al., 4 Aug 2025).

The payload design papers report that the first natural frequency is approximately 270 Hz, well above the 100 Hz minimum requirement, and that stress and displacement levels remain below material limits in the analyzed configurations (Lombardi et al., 4 Aug 2025). The same studies describe topological optimization of the payload–platform interface and of the test fixture, motivated by the need to suppress non-flight-like resonances and reproduce realistic mechanical boundary conditions during laboratory qualification (Lombardi et al., 4 Aug 2025). A mechanical demonstrator and a preliminary environmental test campaign including vibration and shock tests are part of the Phase B plan (Lombardi et al., 4 Aug 2025).

Mission-status reporting in 2025 places CUSP in Phase B, notes that the System Requirements Review was complete in July 2025, and states that the Preliminary Design Review was scheduled for December 2025 (Angelis et al., 16 Sep 2025). The same set of papers gives a launch target of late 2027 or early 2028, conditional on Phase B closure and subsequent development phases (Fabiani et al., 1 Aug 2025). The planned mission duration remains 3 years, with explicit attention to de-orbit compliance within 5 years and a collision probability < $Z$5 with objects larger than 1 cm (Fabiani et al., 1 Aug 2025).

Within heliophysics, the intended significance of CUSP is the provision of hard X-ray polarization measurements capable of constraining flare magnetic geometry, electron anisotropy, and the partition between thermal and non-thermal emission channels (Fabiani et al., 1 Aug 2025). Within spacecraft instrumentation, the mission serves as a case study in deploying a Compton polarimeter on a CubeSat platform with a simulation-calibrated treatment of geometric systematics, coincident-event selection, and minute-scale MDP performance. A plausible implication is that the mission’s most consequential contribution will depend not only on raw sensitivity, but also on the extent to which the Phase B prototype, calibration, and environmental qualification results preserve the low systematic floor assumed by the Geant4 performance models.