The PRIMA Far-Infrared Probe: Observatory and Instrumentation

An actively-cooled telescope optimized for far-IR wavelengths is poised to addresses a wide range of astrophysics questions ranging from galaxy evolution to planetary system formation. Tremendous advances are now possible at wavelengths between JWST and the ground-based platforms thanks to improvements in the detector array technology over the last 2 decades. When cooled and on orbit with suitable detector arrays, even a Herschel- or SOFIA-sized telescope can offer observing speed improvements of some 4 orders of magnitude. Astro2020 recognized this opportunity; the far-IR was identified as one of the two options to be considered for NASA’s first astrophysics probe to launch near 2030. We are developing the PRobe far-Infrared Mission for Astrophysics, (PRIMA), an actively-cooled space-borne observatory to meet this longstanding scientific need. A broad ensemble of scientific themes drive the design, ranging from deep extragalactic spectroscopy to decode the cosmic history of nucleosynthesis, star formation, and supermassive black-hole growth, to detailed study of water and chemical properties of protoplanetary disks throughout their evolutionary sequence. PRIMA combines a passive thermal design in an earth-sun L2 halo orbit with closed-cycle coolers to support a 2-3 meter telescope cooled to 4 K, and instruments and focal planes cooled to 1 K and below. The telescope includes a field steering mirror for modulation and creation of small maps, and the observatory will be agile to enable wide-field scan mapping as well as for quickly slewing to targets of interest within the field of regard. The primary workhorse instrument is a dispersive direct-detection spectrograph with resolving power between 100 and 300, operating near the fundamental sensitivity limit set by the zodiacal light background. It will cover from 25 to at least 200 microns, with a goal of reaching the onset of the ground-based submm windows at 330 microns. This full range will be covered with 4 or 5 wideband grating spectrometer modules operating simultaneously, each fed with a long slit; this architecture provides both optimal pointed object sensitivity as well as excellent spectral mapping speed. The spectrometer will also be equipped with a mode providing spectral resolving power of several thousand, also covering the full band with incurring only modest sensitivity penalty. A multi-purpose imaging experiment is also under study for PRIMA, its potential applications include narrow-band spectrophotometry over wide fields at the short far-IR wavelengths, and dust polarimetry in bands to be optimized in the course of the study.