PRIMA: Far-IR Mission for Astrophysics

• PRIMA is a next-generation far-infrared mission featuring a cryogenically cooled 1.8-m telescope and advanced KID detector arrays.
• Its innovative instruments, PRIMAger and FIRESS, deliver ultra-sensitive imaging and versatile spectroscopy to probe dust properties, galaxy evolution, and interstellar magnetic fields.
• The mission’s community-driven GO program and rapid survey strategy pave the way for transformative studies from stellar formation to cosmic dust mineralogy.

The Probe Far-IR Mission for Astrophysics (PRIMA) is a next-generation space observatory concept focused on ultra-sensitive imaging and spectroscopy in the far-infrared (far-IR), targeting the wavelength interval from 24 to 235 μm. Its architecture is built around a cryogenically cooled 1.8-meter telescope with two principal instruments: a wide-field imager (PRIMAger) and a versatile spectrometer (FIRESS). Designed to deliver mapping speeds and sensitivities orders of magnitude above prior far-IR facilities, PRIMA is structured as a survey and discovery machine with a dominant General Observer (GO) program, aimed at enabling a comprehensive set of astrophysical investigations across topics such as the lifecycle of dust, galaxy and black hole co-evolution, interstellar magnetic field mapping, planetary system formation, and mineralogy of cosmic dust. Its community-driven science program is set to shape the mission’s legacy and maximize discovery potential over its operational life (Moullet et al., 2023).

1. Mission Architecture and Instrumental Overview

The core of the PRIMA mission is a cryogenically cooled 1.8 m primary mirror with two major science instruments:

• PRIMAger: An infrared camera providing two imaging modes:
  • Hyperspectral mode: 24–84 μm coverage with linear variable filters, achieving R = λ/Δλ ≳ 8 spectral resolution. Enables spectro-photometric mapping for diagnostics such as PAH features and fine-structure lines.
  • Polarimetric mode: Four broad bands between ~80 and 264 μm, with R ≈ 4, optimized for linear polarization measurements. Pixels are designed with discrete polarization angles (in sets with 120° separation), allowing full Stokes I, Q, U recovery in a single scan; the full array leverages kinetic inductance detector (KID) arrays operated at ~125 mK for background-limited sensitivity (Ciesla et al., 1 Sep 2025, Dowell et al., 25 Apr 2024).
• FIRESS (Far-InfraRed Enhanced Survey Spectrometer): A slit-fed grating spectrometer covering 24–235 μm via four modules, each feeding a ~24×84 KID array. FIRESS provides flexible operational modes: moderate spectral resolution (R ≈ 85–150) for wide-field mapping and low-res point source work, and high-resolution (up to R = 20,000 at short wavelengths via a Martin–Puplett Fourier transform module) for spectrally demanding science such as dynamical studies and disk chemistry. All detector arrays are read out simultaneously for multiplex advantage (Pontoppidan et al., 1 Sep 2025, M. et al., 2 Sep 2025).
• PRIMA includes a two-axis beam-steering mirror, inherited from Herschel/PACS, which enables efficient cross-linked mapping, high scan speeds, and fine spatial sampling in both imaging and spectroscopy (Dowell et al., 25 Apr 2024).

2. Detector Technology and Sensitivity

PRIMA’s science payload is based on kilo-pixel arrays of kinetic inductance detectors (KIDs), primarily aluminum absorbers coupled via lenslets and low-loss superconducting niobium interdigitated or parallel plate capacitors. The technology delivers several advances:

• Noise equivalent power (NEP): Array pixels reach NEP ≤ 1×10⁻¹⁹ W/Hz¹/², i.e., at or near the photon background limit for the expected low zodiacal and telescope backgrounds. Majority pixel yields >90% have been demonstrated on 1k-pixel arrays at 210 μm, with 73% of pixels achieving NEP < 1×10⁻¹⁹ W/Hz¹/² (Foote et al., 2023, Hailey-Dunsheath et al., 2023).
• Dynamic range: Prototypes offer photon noise–limited sensitivity over incident optical loads from 0.01 aW to ≥20 fW, critical for both faint background-limited imaging and high-flux source spectroscopy.
• Design features: Compact, frequency-multiplexable architecture, minimized two-level system (TLS) noise via parallel plate capacitor designs, optimized absorber geometry, microlens hybridization for efficient optical coupling, and resilience to 1/f noise enabled by destriping map-making (Cothard et al., 2023, Dowell et al., 25 Apr 2024).

The high sensitivity and format KID arrays are directly scalable to the requirements of both PRIMAger and FIRESS and are considered a mature technology basis for the mission (Foote et al., 2023, Hailey-Dunsheath et al., 2023).

3. Scientific Program and GO Survey Framework

PRIMA is characterized by an open science architecture, allocating at least 75% of its operational time over 5 years to a competitive General Observer (GO) program (Moullet et al., 2023). The mission’s unprecedented mapping speed (2–4 orders of magnitude faster than predecessors) supports wide and deep surveys in both intensity and polarization, as well as targeted pointed spectroscopy:

• Example legacy cases:
  • Mineralogy at Cosmic Noon: Surveys using moderate-resolution FIRESS spectroscopy (R ≈ 130) to detect crystalline silicate features (notably the 69 μm forsterite band) in ~130 galaxies at 1.5 ≤ z ≤ 4, enabling measurement of the interstellar “crystalline fraction”—a key diagnostic of dust processing and lifecycle during the peak of cosmic star formation (Moullet et al., 2023).
  • Galaxy and SMBH evolution: Large photometric and spectroscopic surveys targeting dusty galaxies, AGN, and star formation up to z ≃ 4, using the full “infrared toolkit” (continuum SEDs, PAHs, fine-structure lines) to disentangle star formation and black hole accretion histories (Bisigello et al., 26 Apr 2024, Faisst et al., 1 Sep 2025).
  • Polarimetric mapping: Mapping the ordered and turbulent magnetic fields in the ISM and extragalactic environments via polarization imaging in four bands. The instrument design enables high-fidelity, low-systematic recovery of Stokes parameters (I, Q, U) and magnetic field orientation with sensitivity and resolution unmatched by SOFIA/HAWC⁺ (Dowell et al., 25 Apr 2024, Maglione et al., 2 Sep 2025, Paré et al., 14 Mar 2025).
• Survey strategy: Surveys of hundreds to thousands of galaxies, with integration times tailored to reach required line or continuum sensitivities on the scale of 1–10 hours per target. Survey speed and mapping depth are optimized relative to confusion and instrumental noise floors.

4. Observational Methodologies and Data Analysis

• Spectral and spatial mapping: PRIMA employs both slit-scan mapping (FIRESS), and large-area intensity and polarization mapping (PRIMAger) with advanced scan strategies (two-axis BSM, cross-linked scans). The spectral coverage from 24 to 235 μm enables simultaneous sampling of dust SEDs and solid-state features (e.g., forsterite).
• Polarimetric measurement: Use of three-angle detector sets allows instantaneous measurement of linear polarization without the need for a rotating half-wave plate. Destriping map-making, proven in Herschel/SPIRE and Planck, is carried over for baseline and 1/f noise removal. Dedicated calibration sources and internal analysis allow for precise pixel gain calibration and mitigation of systematic effects (e.g., beam ellipticity, pointing error).
• Source extraction in confusion: Confusion limits, predominantly at longer wavelengths (tens of mJy at 235 μm), set the practical depth for blind extraction of galaxies; in polarization, confusion limits are over 100× lower due to low polarization fractions and vector cancellation. Blind source extraction pipelines are validated on simulated maps for completeness, purity, and flux accuracy using formulas such as

$$\sigma_{\rm conf}^2 = \int_0^{S_{\rm lim}} S^2\, \frac{dN}{dS}\, dS$$

where the integration is over sources fainter than the detection threshold $S_{\rm lim}$ (Béthermin et al., 5 Apr 2024).

• Survey synergy: Short-wavelength (higher resolution, lower confusion) intensity data are used to deblend longer-wavelength data, while deep polarization mapping at longer wavelengths provides unique independent information on dust and magnetic fields. This synergy is intrinsic to the PRIMAger design and opens new parameter spaces in high-redshift dust polarization studies (Béthermin et al., 5 Apr 2024).

5. Key Scientific Impact

PRIMA’s combination of high sensitivity, full spectral coverage, and large survey speed is expected to enable:

• Crystalline fraction of dust at high redshift: Capability to resolve and quantify crystalline silicate features in the ISM of star-forming galaxies at Cosmic Noon, advancing the understanding of the dust production, destruction, and processing cycle and providing major constraints on models of interstellar grain evolution (Moullet et al., 2023).
• Calibration of dust lifecycle and energy distributions: Measurement of the persistence of crystalline dust versus amorphization in noisy, high-SFR environments, with implications for SED modeling, interpretation of high-z galaxy photometry, and chemical enrichment.
• Interstellar magnetic field mapping: Deep maps of I, Q, U at <30″ spatial resolution allow paper of turbulence, alignment, and the interplay of magnetic fields and cloud structure in both the Milky Way’s center and extragalactic systems.
• Community science and unexpected discovery: The open design and GO allocation, with advanced detector technology and flexible operational modes, maximize the potential for new discoveries across the astrophysical parameter space.

6. Technical and Programmatic Context

• Technological advances: PRIMA leverages decade-long technical progress in KID/microlens arrays, cryogenic telescope design, and instrument multiplexing. Detector performance meets or exceeds the NEP specification required for background-limited far-IR space missions.
• International collaboration: Development involves French institutions (LA Marseille, CEA, CNES), SRON (Netherlands), Cardiff University (UK), and NASA/JPL/GSFC (US), ensuring technical and scientific breadth (Ciesla et al., 1 Sep 2025).
• Confusion and limitations: While instrumental sensitivity is background-limited, ultimate survey depth at longer λ is limited by confusion noise, not instrumental noise; this constrains the faint end for galaxy source extraction in the FIR.
• Data products: PRIMA will deliver large, well-characterized datasets: intensity maps, polarization maps, and spectral cubes across 24–235 μm, with accompanying catalogs containing derived properties (e.g., crystalline fraction, SFR, AGN indicators).

7. Broader Implications for Astrophysics

PRIMA’s enabling design addresses limitations of previous missions (e.g., Spitzer, Herschel, JWST) by offering a unique combination of wavelength coverage, sensitivity, and survey speed in the far-IR. It directly targets questions left open by the mid-IR cutoff of JWST (28 μm) and the spectral/coverage restrictions of its predecessors, making it the first facility able to systematically paper dust mineralogy, star formation, AGN activity, and magnetic fields at the epoch of peak star formation and beyond. The mission’s science book structure anticipates substantial contributions to fields ranging from galaxy formation and evolution to condensed matter astrophysics, all under a model enabling broad community-driven science (Moullet et al., 2023).