We present the science drivers for the far-infrared enhanced survey spectrometer (FIRESS), one of two science instruments on the PRobe Infrared Mission for Astrophysics. FIRESS is designed to meet science objectives in the areas of the origins of planetary atmospheres, the co-evolution of galaxies and supermassive black holes, and the buildup of heavy elements in the universe. In addition to these drivers, FIRESS is envisioned as a versatile far-infrared spectrometer, capable of addressing science questions in most areas of astrophysics and planetary astronomy as part of a dominant General Observer (GO) program with 2/3 of the current science cases using FIRESS. We summarize how the instrument design choices and parameters enable the main science drivers as well as a broad and vibrant GO program.

The Astro2020 Decadal Survey recommended a new line of astrophysics observatories intermediate in scale between MIDEXs and Flagship-class observatories. In response, NASA created the Astrophysics Probe Explorer class and solicited proposals for the first generation of Probes. With a larger cost cap, Probes can achieve more ambitious science than SMEXs or MIDEXs and be implemented faster than Flagships—as frequently as one per decade. The PRobe far-Infrared Mission for Astrophysics (PRIMA) is one of two Probe concepts selected by NASA for a concept study in 2024/2025, potentially leading to implementation and launch as early as 2031. PRIMA was designed for a broad range of astrophysics, from how planets assemble their atmospheres, to the coevolution of galaxies and black holes, to the evolving properties of dust and galactic metallicity over cosmic time. Seventy-five percent of PRIMA’s observing time will be allocated to guest observer observations and 25% allocated to principal investigator science; however, the principal investigator science data will be available promptly for guest investigator usage. The observatory features a 1.8-m diameter telescope cooled to 4.5 K with two science instruments: the Far-InfraRed Enhanced Survey Spectrometer (FIRESS) and the PRIMA imager (PRIMAger). FIRESS provides continuous spectral coverage from 24 to 235 μm, in two spectral resolution modes (R ≥ 85 and R ¼ 4400ð112 μm∕λÞ), with spectral mapping capability and order-of-magnitude sensitivity improvement over previous observatories. PRIMAger delivers similar sensitivity advances and first-of-its-kind far-infrared hyperspectral imaging for astrophysics with R ∼ 8 from 25 to 84 μm, and polarimetry in four broadband filters from 80 to 261 μm. PRIMA’s science and technical motivation is outlined, its overall architecture is described, and its cryogenic payload and instruments, including the kinetic inductance detector arrays, and operations and observing modes, are summarized.