Alcor Program: Astrometry & CubeSat Missions

• Alcor Program is a multifaceted initiative by the Italian Space Agency that advances high-precision astrometry and CubeSat-based missions for space science.
• It employs a pupil-plane grid and common parallax techniques to achieve rapid, milliarcsecond-precision detection of stellar companions.
• The Program also supports miniaturized instruments like the CUSP mission, which measures solar flare polarization with high sensitivity for space weather research.

The Alcor Program is a multi-faceted initiative facilitated by the Italian Space Agency, encompassing efforts to advance both astronomical companion detection methods and CubeSat-based space science missions. Its domain includes the application of high-precision astrometric techniques to stellar multiplicity studies—exemplified by the discovery of an M-dwarf companion to the A5V star Alcor—and the technological development and deployment of miniaturized space instruments for astrophysical and space weather research, notably through the CUSP (CUbesat Solar Polarimeter) mission.

1. Genesis and Framework

The Alcor Program originated as an effort to bridge gaps in detection, confirmation, and characterization of faint stellar and substellar companions around bright nearby stars, while simultaneously promoting innovation in miniature space observatories, particularly CubeSats. Its framework integrates advanced ground-based astronomical instrumentation with targeted space missions, benefiting both the paper of stellar astrophysics and the technological progress of small-scale space hardware.

A seminal activity under the Program was the Project 1640 effort that utilized novel pupil-plane astrometry to enable rapid confirmation of stellar companions by measuring common parallactic motion. Separately, within the field of in-orbit instrumentation, the Program established itself as a funding and technology incubator for scientific CubeSat missions, most notably approving and supporting the CUSP mission for hard X-ray solar flare polarimetry (Fabiani et al., 1 Aug 2025, Fabiani et al., 2022). The Program’s reach, thus, spans both high-contrast companion detection and small satellite-based astrophysical measurement.

2. High-Precision Astrometry for Companion Detection

A flagship result of the Alcor Program was the detection and spectrophotometric analysis of an M3–M4 dwarf companion to Alcor, enabled by a combination of adaptive optics coronagraphy, imaging spectroscopy, and a novel astrometric pupil plane grid (0912.1597). The core technical innovation consists of a grid—formed by thin opaque lines in the coronagraph’s pupil plane—imprinting a predictable array of so-called “satellite spots” on the focal plane image. The spacing of these spots, set by $\lambda/d$ (with $\lambda$ the observation wavelength and $d$ the projected grid spacing), and their intensity, proportional to $(t/d)^2$ (where $t$ is line thickness), provide a robust positional fiducial for the occulted primary.

This arrangement enabled determination of the relative separation between Alcor and the candidate companion with $\leq 3$ milliarcsecond uncertainty—a critical threshold for reliable astrometry at the $\sim$1" separation and $\sim$6 magnitude contrast encountered in the system. Combined with the Project 1640 instrument’s integral field spectrograph capability (spanning 1.10–1.76 μm, $R \sim 30$) and adaptive optics delivering Strehl ratios of $\sim 0.5$ at $1.65\, \mu$m, the approach yielded both spatial and spectral information on the companion.

3. Common Parallax as a Rigorous Companionship Test

A key methodological development under the Alcor Program was confirmation of physical association using common parallactic displacement observed over a 103-day baseline (0912.1597). In systems at $\sim$25 pc, the backdrop of significant annual parallax allows differentiation between chance alignments with background sources and true bound companions in a matter of months.

For the Alcor companion:

• A stationary background source would exhibit a shift of $\sim$34 mas relative to Alcor over the 103 days, owing to parallax and proper motion effects.
• The measured change for the point source (companion) was limited to $-6.0\pm4.3$ mas (east), $+10.9\pm2.3$ mas (north), consistent with small orbital motion rather than the expected background track.
• The full measured displacement is modeled as $\Delta_{\rm PM+\pi} = \Delta_{\rm PM} + \Delta_\pi$, combining proper motion and parallax vectors.

This “common parallax” approach is more robust than traditional reliance on common proper motion, as it directly validates co-location at the same distance, not just co-moving sky projection. A plausible implication is the reduction in temporal baseline required for confirmation of new companions in high-precision astrometric imaging campaigns.

4. Expansion of Binary Star Parameter Space

The Alcor Program’s astrometric approach facilitated exploration of binary star configurations previously underrepresented in observational surveys. Discovery of a $\sim 0.25\,M_\odot$ M-dwarf companion to a luminous, nearby A5V star at $d=25$ pc (V=4.0) probed a parameter regime of high-priority for constraining stellar formation pathways (0912.1597). These results also impact the interpretation of anomalous X-ray emissions in so-called “X-ray-loud” A stars, supporting the hypothesis that such fluxes may be attributed to unseen, magnetically-active cool companions rather than intrinsic processes.

The prompt astrometric confirmation via common parallax may encourage future high-contrast imaging surveys to systematically reappraise binarity and multiplicity statistics, especially in domains where mass ratios are extreme and contrast a limiting factor.

5. CubeSat Development and Mission Design: The CUSP Project

Within the framework of fostering small-scale scientific missions, the Alcor Program presided over the selection, maturation, and technical advancement of the CUSP (CUbesat Solar Polarimeter) project (Fabiani et al., 2022, Fabiani et al., 1 Aug 2025). Designed as an Earth-orbiting, dual-satellite constellation, CUSP’s primary objective is to measure the linear polarization of hard X-ray flare emission (20–100 keV) from the Sun—providing direct probes of magnetic reconnection and particle acceleration.

Distinctive mission features include:

• Deployment of two CubeSats in the same orbital plane at $180^\circ$ phase, ensuring uninterrupted solar monitoring.
• A dual-phase, Compton-scattering polarimeter: plastic scintillator for initial scatter, GaGG (Gd$_3$Al$_2$Ga$_3$O$_{12}$) crystal for absorption, read out by Multi-Anode PMTs and avalanche photodiodes, with event selection and processing managed by MAROC 3A and SKIROC 2A ASICs.
• Operational rotation at $\sim 1$ RPM to average out systematic instrument modulation, enhancing the reliability of polarization measurements.

Rigorous Phase B activities, underwritten by the Program, include detailed mission analysis and comprehensive compliance with debris mitigation (orbital decay within 5 years, collision probability $<10^{-3}$ for $>1$ cm objects).

6. Scientific Rationale: X-ray Polarimetry of Solar Flares

The fundamental measurement principle employed by CUSP is Compton polarimetry. The instrument utilizes the anisotropy of the Compton scattering cross section for polarized photons, described by the Klein-Nishina formula: $$\frac{d\sigma}{d\Omega} = \frac{r_0^2}{2} \left(\frac{E'}{E}\right)^2 \left[ \frac{E}{E'} + \frac{E'}{E} - 2 \sin^2 \theta \cos^2\phi \right]$$ where $E$ and $E'$ are incident and scattered photon energies, $\theta$ is the polar angle, and $\phi$ is the azimuth from polarization direction. For maximally polarized, hard X-ray emission (e.g., impulsive phase non-thermal Bremsstrahlung), the modulation factor

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

quantifies the contrast between azimuthal maxima/minima in the event histogram. The Minimum Detectable Polarization (MDP) sets sensitivity limits: $$\mathrm{MDP}_{99\%} = \frac{4.29}{\mu R} \sqrt{\frac{R+B}{T}}$$ where $R$ is source count rate, $B$ background, and $T$ integration time. CUSP estimates MDP values as low as 3.9% for X1.2-class flares with $\sim5$ minutes of observation (Fabiani et al., 1 Aug 2025).

These data can discriminate between thermal (weakly polarized) and non-thermal (strongly polarized) emissions, constraining theories of energy release and electron acceleration during solar flares.

7. Broader Impacts and Prospects

The scientific and technological activities under the Alcor Program yield several impacts:

• Improvement in the parameterization and detection efficiency for binary and multiple star systems, particularly those with high contrast and small angular separation.
• Demonstration that CubeSat-scale missions can meaningfully contribute to high-energy astrophysics and space weather monitoring (e.g., integration of data with frameworks such as ASPIS).
• Advancement of CubeSat payload TRL for polarimetric instrumentation, with CUSP pivoting from TRL 3 (component/analytical demonstration) to TRL 7 (system prototype demonstration in operational environment).

A plausible implication is that the Alcor Program’s model, combining cutting-edge instrumentation, advanced astrometric techniques, and agile space platform development, could be generalized to similar national and international initiatives seeking efficient, scalable astrophysical observation strategies.

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Summary Table: Selected Features and Results of the Alcor Program

Activity	Domain	Key Innovation
Project 1640 (Alcor companion imaging)	Astrometry	Pupil-plane grid for common parallax confirmation ($\leq 3$ mas precision)
CUSP (CUbesat Solar Polarimeter)	Space CubeSats	Dual-phase Compton polarimeter, two-satellite constellation, $<4\%$ MDP for strong flares
Space Weather Integration (e.g., ASPIS)	Applications	Real-time polarimetric data from CubeSats for improved forecasting

The Alcor Program thus constitutes a key reference point for advanced stellar astrophysics and the future of distributed, low-cost space science missions.