---
title: 'CUSP: CubeSat Solar Polarimeter'
url: https://www.emergentmind.com/topics/cusp
type: topic
---

# CUSP: CubeSat Solar Polarimeter

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** [2509.13104]. 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 [2508.00661].

## 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 [2508.00661]. 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** [2509.13104].

The observational strategy is oriented toward **time-resolved measurements** of polarization degree and polarization angle on timescales ranging from **minutes to hours** [2508.00661]. 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 [2508.00661].

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 [2508.00661]. 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 [2508.00661]. 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 [2508.00661]. 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 [2508.00661].

The orbit analysis in the mission overview identifies a **Sun-Synchronous Orbit** at approximately **500–525 km altitude** as the reference solution [2508.00661]. 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 [2508.00661]. The mission papers quote **<2°** attitude precision during science mode and **<0.04° @ 3\(\sigma\)** absolute knowledge error in the technical specification set [2508.00661].

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 [2208.06211]. 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 [2508.00661]. 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** [2508.00661].

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** [2508.00661]. 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** [2508.00661].

## 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 [2509.13104]. 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** [2509.13104]. 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 [2508.00642].

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** [2509.13104]. 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 [2509.13104].

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 [2508.00661]. 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 [2508.02594]. 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, \(\theta\) is the polar scattering angle, and \(\phi\) is the azimuthal angle measured relative to the polarization vector [2508.00661]. 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** \(\mu\), the **quality factor** \(Q\), and the **Minimum Detectable Polarization** (MDP). The mission overview defines

$$
\mu(\theta)=\frac{N_{\max}-N_{\min}}{N_{\max}+N_{\min}},
$$

and

$$
Q=\mu\sqrt{\epsilon},
$$

where \(\epsilon\) is the detector efficiency [2508.00661]. For statistical sensitivity at **99% confidence**, the same paper gives

$$
\mathrm{MDP}_{99\%}=\frac{4.29}{\mu R}\sqrt{\frac{R+B}{T}},
$$

with \(R\) the source count rate, \(B\) the background count rate, and \(T\) the observing time [2508.00661].

The Geant4 performance paper adds a **Stokes-parameter** formulation for event-by-event analysis. For event \(i\), it defines

$$
q_i = 2\cos(2\phi_i), \qquad u_i = 2\sin(2\phi_i),
$$

with normalized sums

$$
q=\frac{1}{N}\sum_i q_i, \qquad u=\frac{1}{N}\sum_i u_i,
$$

and modulation amplitude

$$
m=\sqrt{q^2+u^2}.
$$

For a fully polarized beam, \(m=\mu_{100}\), while the inferred polarization degree and angle are obtained from

$$
p=\frac{m}{\mu_{100}}, \qquad \varphi=\frac{1}{2}\tan^{-1}\left(\frac{u}{q}\right).
$$

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 [2508.00780].

## 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 [2508.00780]. 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 [2407.04134].

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 [2508.00780]. 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 [2508.00780]. 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 [2508.00780].

The expected polarimetric sensitivity reported in the mission-overview literature is summarized below for the **25–100 keV** band at **99% confidence** [2508.00661].

| 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 [2508.00661]. 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 [2509.13104].

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 [2508.00642]. The reported radioactive sources include **\(^{55}\)Fe, \(^{241}\)Am, and \(^{109}\)Cd** for the scatterer calibration, and **\(^{241}\)Am, \(^{109}\)Cd, \(^{57}\)Co, \(^{155}\)Eu, and \(^{129}\)I** for the absorber calibration [2508.00642]. 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 [2508.00642].

## 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 [2508.02594]. 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 [2508.02594]. 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 [2508.02594].

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 [2508.02594]. 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 [2508.02594]. A **mechanical demonstrator** and a preliminary environmental test campaign including **vibration** and **shock** tests are part of the Phase B plan [2508.02594].

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** [2509.13104]. The same set of papers gives a launch target of **late 2027 or early 2028**, conditional on Phase B closure and subsequent development phases [2508.00661]. The planned mission duration remains **3 years**, with explicit attention to **de-orbit compliance within 5 years** and a collision probability **< \(10^{-3}\)** with objects larger than **1 cm** [2508.00661].

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 [2508.00661]. 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.

Source: https://www.emergentmind.com/topics/cusp