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Astronomy at far-infrared (far-IR) wavelengths is essential to our understanding of the evolution of the cosmos, from the star formation history of galaxies to how the ice distribution affects the formation of extrasolar planetary systems. The Hubble Space Telescope, James Webb Space Telescope, and the Atacama Large Millimeter Array have already produced ground-breaking astronomical observations with high angular resolution spanning the visible to sub-millimetre wavelength regimes. However, this presents a gap in the far-IR, from roughly 30−400μm, where ground-based observations are largely intractable due to the opacity of Earth’s atmosphere. Indeed, no telescope, observatory, or interferometry array has ever achieved sub-arcsecond angular resolution over this wavelength range. A space-based solution is needed. However, a space-based far-IR telescope capable of subarcsecond angular resolution and high sensitivity, at a cost comparable to the largest space missions to date, presents unique physical, practical, and engineering challenges. In this paper, we envisage what a far-IR Great Observatory class mission might look like in the context of the already-studied Origins Space Telescope (OST) and the Space Infrared Interferometric Telescope (SPIRIT). We begin with a historical reflection of far-IR missions, including OST and the recommendations by the Astro2020 Decadal Survey for a de-scoped mission. We use this to motivate the recommendation of a space-based interferometer as a reasonable path towards sub-arcsecond angular resolution at far-IR wavelengths. Using the SPIRIT mission concept as inspiration, we consider multiple point designs for a two element, structurally connected spatial-spectral space-based far-IR interferometer to understand the implications on achieved angular resolution and estimate total mission cost in context of the Decadal Survey recommended far-IR Great Observatory cost cap. This paper illustrates the unique capabilities only possible through a space-based far-IR double Fourier interferometry mission capable of sub-arcsecond resolution.
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