Mini-EUSO: ISS UV Detector for Cosmic Rays

Updated 23 January 2026

Mini-EUSO is a space-based UV telescope on the ISS designed to detect UHECR air showers and validate fluorescence detection techniques.
It employs fast photon-counting with multi-level trigger logic to produce high-resolution global UV maps and analyze transient atmospheric phenomena.
The mission studies meteors, TLEs, and exotic particles, offering crucial insights for the design of next-generation orbital observatories.

Mini-EUSO (Multiwavelength Imaging New Instrument for the Extreme Universe Space Observatory) is the first space-based detector of the JEM-EUSO program, operating since October 2019 on the International Space Station (ISS) with the primary aim of studying ultra-high-energy cosmic rays (UHECRs) via the fluorescence technique and mapping the night-time ultraviolet (UV) emissions of the Earth at high spatial and temporal resolution. The instrument serves as both a pathfinder for future orbital observatories (such as K-EUSO and POEMMA) and a multipurpose facility for the observation of atmospheric transients, meteors, and anthropogenic light sources. Its design integrates fast photon-counting, multi-level triggering, and absolute calibration protocols to validate detection concepts under realistic orbital conditions (Casolino et al., 2022, Battisti, 16 Jan 2026, Romoli, 2023).

1. Scientific Objectives and Mission Rationale

Mini-EUSO targets a broad science portfolio, informed by the requirements of large-scale future UHECR missions and key atmospheric, geophysical, and astronomical phenomena. The main objectives are:

Space-based UHECR Search and Technology Validation: Detect extensive air showers (EAS) induced by UHECRs above $E_{\rm th}\gtrsim10^{21}$  eV, test multi-level trigger logics under real backgrounds, establish upper limits for the flux at energies above the GZK cutoff, and provide in-orbit calibration for fluorescence detection techniques (Casolino et al., 2022, Romoli, 2023, Battisti, 16 Jan 2026).
Global Mapping of Night-time UV Emission: Produce the first all-sky data set of night-side UV radiance in the 290–430 nm band, resolving features from airglow, anthropogenic light, cloud reflection, and bioluminescence at 6.3 km spatial and 40 ms temporal resolution (Casolino et al., 2022, Casolino et al., 2022).
Atmospheric and Transient Phenomena: Capture and analyze transient luminous events (TLEs)—notably ELVES, sprites, and halos—with microsecond time tagging; perform systematic meteor surveys including interstellar candidates; and characterize short light transients (SLTs) in potential overlap with UHECR events (Marcelli et al., 2021, Romoli, 2023, Battisti, 16 Jan 2026).
Exotic Particle Search: Search for nuclearites (hypothetical strange quark matter) via persistent, high-speed tracks distinct from ordinary meteors or artificial satellites (Casolino et al., 2022).
Benchmarking for Future Missions: Quantify orbital duty cycles, validate end-to-end calibration with ground UV flashers and lasers, and derive exposure/detection thresholds to inform design trade-offs for K-EUSO and POEMMA (Casolino et al., 2022, Bertaina et al., 15 Nov 2025, Bertaina et al., 2023).

2. Instrumentation, Optical and Electronic Architecture

The Mini-EUSO payload is a compact, fully enclosed UV telescope (dimensions $37\times37\times62$ cm $^{3}$ , mass ≃35 kg, power ≃55–60 W) installed on the zenith-facing UV-transparent window of the Zvezda module on the ISS (Casolino et al., 2022, Cambié et al., 2022, Marcelli, 2023). Its technical architecture encompasses:

Optical System: Two PMMA Fresnel lenses, each 25 cm in diameter, focal length ≈300 mm, form a wide-aperture ( $\approx0.05$ \;m $^{2}$ ) refractive telescope optimized for throughput (50–80%) in 290–430 nm (BG3 filter bandpass). The field of view is a $44^\circ \times44^\circ$ square ( $\sim$ 350 km $\times$ 350 km ground imprint), with per-pixel angular resolution $\Delta\theta\approx0.92^\circ$ ( $\sim$ 6.3 km at 400 km ISS altitude) (Casolino et al., 2022, Casolino et al., 2022, Cambié et al., 2022).
Photo-Detector Module (PDM): 36 Hamamatsu R11265-M64 Multi-Anode PMTs (MAPMTs), each with 8 $\times$ 8 pixels (total 2,304 channels), directly coupled to the focal plane for single-photon counting. The end-to-end quantum efficiency is $\epsilon(\lambda)\gtrsim50\%$ across the band (Casolino et al., 2022, Battisti, 16 Jan 2026).
Electronics/Trigger Logic: Readout at $2.5\,\mu\mathrm{s}$ $2.5 μ s$ (GTU) enables three concurrent acquisition channels:
- D1: $2.5\,\mu$ s for UHECR and fast TLEs—hardware triggers require $16\sigma$ above the dynamic mean persisting $\geq8$ GTU.
- D2: $320\,\mu$ s for slower transients.
- D3: $40.96$ ms continuous frames for mapping and slow tracks (meteors, nuclearites) (Casolino et al., 2022, Belov et al., 2017, Romoli, 2023).
- Onboard logic is implemented on a Xilinx Zynq FPGA, with multilevel background estimation and dynamic per-pixel gain/veto control to protect against sensor saturation (Belov et al., 2017, Marcelli, 2023, Cambié et al., 2022).
Ancillary Sensors: Co-aligned visible (400–780 nm) and near-IR (1,500–1,600 nm) cameras deliver contextual imaging for atmospheric monitoring and event source discrimination (Casolino et al., 2022, Golzio et al., 2021).

3. Data Acquisition, Calibration, and Photometric Methodology

Data are buffered and stored onboard via solid-state media, with typical sessions of $\sim$ 12 h and total science exposure exceeding 750 h as of late 2025 (Battisti, 16 Jan 2026). Calibration strategy includes:

Flat-Fielding: Pixel gain variation and vignetting are corrected by identifying “minimum-light” bins (from cloud/ocean overpasses) in each session, and normalizing raw counts to a reference minimum per pixel. Absolute calibration leverages ground UV flasher campaigns and cross-instrumental dark runs, with in-orbit average pixel detection efficiency $\bar{\epsilon}=7.3\%$ (at 400 nm) (Casolino et al., 2022, Battisti, 16 Jan 2026, Miyamoto et al., 2021).
Radiance Conversion: Count rates $C$ (per 2.5 μs GTU) are converted to physical photon fluxes $\phi$ [ph cm $^{-2}$ sr $^{-1}$ s $^{-1}$ ] using the relation

$\phi_{\rm ground} = \frac{C}{\epsilon\,A\,\Omega\,{\rm Atm}} \approx C\times(550\pm100)\,\mathrm{ph\,ns^{-1}\,m^{-2}\,sr^{-1}}$

for extended ground sources, and analogous expressions for atmospheric emissions and pointlike flashes (Casolino et al., 2022, Shinozaki et al., 2021).

Event Geolocation and Environmental Association: Each pixel and frame is georeferenced to ISS position, synchronized with meteorological data (e.g., Global Forecast System cloud masks), and mapped into 0.1° × 0.1° Earth cells for global, regional, and local analysis (Casolino et al., 2022, Golzio et al., 2021).

4. Key Scientific Results: Observational Products and Phenomena

4.1. Night-side UV Maps

Mini-EUSO produced the first global UV radiance maps with $\sim6.3$ km spatial resolution at $40.96$ ms temporal sampling, resolving both natural and anthropogenic emission features. Key results include:

Baseline Backgrounds: $0.9\pm0.4$ cts/pix/GTU over clear ocean, $1.4\pm1.6$ cts/pix/GTU over land on dark, moonless, cloud-free nights; radiances $\phi\sim500\pm100$ ph ns $^{-1}$ m $^{-2}$ sr $^{-1}$ for airglow; anthropogenic hotspots (cities, fishing fleets) up to $10–20$ cts/pix/GTU (Casolino et al., 2022, Casolino et al., 2022).
Effects of Atmospheric Conditions: Clouds increase background linearly by $\sim0.011$ cts/GTU per 1% cloud, while moonlight induces a phase- and zenith-dependent background enhancement parameterized by $I(m,\theta)$ (Casolino et al., 2022, Golzio et al., 2021).
Spatial Structure: Continental boundaries, city light profiles, and marine features (e.g., bioluminescent blooms) are resolved quantitatively (Casolino et al., 2022, Shinozaki et al., 2021).

4.2. Transient Luminous Events (TLEs)

Mini-EUSO is uniquely suited to observing ELVES—rapidly expanding, ring-shaped emissions at $\sim$ 90 km associated with lightning EMPs—due to its fast acquisition and favorable geometry:

Observation Statistics: 37 ELVES in 160 h (2019–2022), including structures with up to five concentric rings. Radii up to $\sim800$ km and durations spanning 100–350 μs are reconstructed with $<5$ % radius-fitting error (Battisti, 16 Jan 2026, Romoli, 2023, Marcelli et al., 2021).
Morphology and Dynamics: Expansion velocity $v\sim c$ consistently measured. Both single and multiple ring events are identified, revealing details of EMP-ionosphere coupling (Romoli, 2023, Marcelli et al., 2021).
Scientific Implications: Systematic mapping of global ELVE occurrence and their correlation to thunderstorm microphysics (Romoli, 2023).

4.3. Meteors and Interstellar Candidates

Mini-EUSO delivers the first comprehensive space-based meteor survey:

Statistics: $>22{,}000$ meteors detected, with rates $\sim2.9$ min $^{-1}$ , magnitude limit $m\sim6$ , speed distribution peaking at $20–60$ km s $^{-1}$ . Three events are classified as interstellar candidates based on $v>71$ km s $^{-1}$ (Battisti, 16 Jan 2026, Marcelli, 2023).
Tracks and Population Diagnostics: D3 mode enables detailed light curves, trajectory mapping, and discrimination against artificial objects or nuclearite candidates (Casolino et al., 2022, Marcelli, 2023).

4.4. UHECR Search and Exotic Particle Limits

No UHECR events were detected above $10^{21}$ eV in $\sim$ 750 h exposure; Poisson 95% C.L. upper limit is $J(E>10^{21}\ \mathrm{eV})<3.5\times10^{-4}$ km $^{-2}$ sr $^{-1}$ yr $^{-1}$ . Mini-EUSO sets leading orbital upper limits for this energy regime (Battisti, 16 Jan 2026, Casolino et al., 2022, Bertaina et al., 2023).

For nuclearites or strange quark matter candidates, persistent, straight tracks are absent; current datasets limit the flux to $\Phi_{90\%}\lesssim1.7\times10^{-20}$ cm $^{-2}$ s $^{-1}$ sr $^{-1}$ (masses $>$ 50 g), with projected sensitivity $<6\times10^{-21}$ over the full mission duration (Marcelli, 2023, Casolino et al., 2022).

4.5. Short Light Transients and Discrimination

Mini-EUSO has demonstrated discrimination between UHECR-like short light transients (SLTs) and atmospheric (TLE or anthropogenic) sources, establishing the critical role of combined temporal/light-curve and spatial footprint analysis to suppress false positives in fluorescence-triggered space missions (Battisti, 16 Jan 2026, Casolino et al., 2022, Miyamoto et al., 2021).

5. Calibration, Environmental Effects, and Cloud Characterization

The response of Mini-EUSO to environmental and instrumental effects is extensively characterized:

Clouds: Cloud reflectance increases UV radiance, detectable as elevated counts in D3 mode. Mini-EUSO can identify low/mid-level clouds with a Heidke Skill Score (HSS) up to $\sim0.4$ in optimal conditions, but is less effective for high, thin cirrus; this necessitates IR and lidar atmospheric monitoring in future missions (Golzio et al., 2021, Casolino et al., 2022).
Absolute Calibration: In-flight tests with ground-based xenon flashers and UV-LED arrays enable full-chain calibration, confirming the validity of physical radiance conversions, and constraining hardware performance (detection efficiency $\epsilon=0.08\pm0.015$ ) (Casolino et al., 2022, Miyamoto et al., 2021, Miyamoto et al., 2021).
Dynamic Range and Background Control: Single-photoelectron sensitivity is maintained up to $\sim200$ cts/pix/GTU; in-flight gain control mitigates the risk from direct lightning or city over-flights (Casolino et al., 2022, Marcelli, 2023).

6. Trigger Logic, Data Handling, and Operational Performance

Mini-EUSO implements a fully parallel, multi-level hardware trigger (D1–D3), validated both in laboratory (e.g., TurLab rotating tank simulations) and in orbit (Belov et al., 2017, Miyamoto et al., 2021):

D1 (2.5 μs): Targets fast, high-significance events (UHECR, ELVES, anthropogenic flashers) with a dynamic noise estimation and 16σ threshold.
D2 (320 μs): Catches millisecond-class phenomena (lightning channels, extended TLEs).
D3 (40.96 ms): Provides continuous coverage for slow phenomena (meteors, long TLEs, mapping).
Data Volume and Throughput: The system is optimized for ISS downlink/storage constraints, with average event rates and data volumes controlled to remain within allocated bandwidth (∼507 kB/s) (Belov et al., 2017).
False Trigger Control and Adaptation: Adaptive per-pixel thresholds, dynamic clustering, and background modeling minimize false triggers, even under rapidly varying albedo or man-made light (Belov et al., 2017, Miyamoto et al., 2021).

7. Legacy, Impact, and Future Mission Design

Mini-EUSO's results have set the reference for the design and operational parameters of next-generation space-based UHECR and atmospheric observatories. Main implications include:

Threshold Scaling: The UHECR threshold for space-based fluorescence telescopes scales as $E_{\rm th}\propto\sqrt{\Omega_{\rm pix}/A_{\rm lens}}$ ; Mini-EUSO's $\sim$ 0.05 m $^{2}$ lens area demands $E_{\rm th}\gtrsim10^{21}$ eV. Increasing the collecting area to $\sim3$ m $^{2}$ and reducing solid angle per pixel allow thresholds to approach $\sim10^{19}$ eV, suitable for large-scale UHECR science (Casolino et al., 2022, Bertaina et al., 15 Nov 2025, Bertaina et al., 2023).
Operational Duty Cycle: The net observational duty cycle (suitable darkness, low background, cloudless) is $\sim$ 20–25%, in agreement with JEM-EUSO projections; detailed duty cycle/UV map analysis guides scheduling for optimal exposure (Casolino et al., 2022, Bertaina et al., 15 Nov 2025).
Calibration and Data Products: Methods pioneered by Mini-EUSO—including in-orbit calibration, cloud monitor synergy, and multi-band mapping—are now baseline for upcoming M-EUSO/POEMMA-like missions (Bertaina et al., 15 Nov 2025, Bertaina et al., 2023, Romoli, 2023).
Open Phenomenology: The combination of high-cadence, high-resolution UV imaging and dynamic triggering proves essential for atmospheric science, planetary defense, astro-particle studies, and multidisciplinary Earth observation (Battisti, 16 Jan 2026, Marcelli, 2023).