  Future far-infrared astronomy missions will need large arrays of detectors
with exceptionally low noise-equivalent power (NEP), with some mission concepts
calling for thousands of detectors with NEPs below a few $\times 10^{-20}$
W/$\sqrt{\mathrm{Hz}}$. Though much progress has been made toward meeting this
goal, such detector systems do not exist today. In this work, we present a
device that offers a compelling path forward: the longitudinal proximity effect
(LoPE) transition-edge sensor (TES). With a chemically-stable and
mechanically-robust architecture, the LoPE TES we designed, fabricated, and
characterized also exhibits unprecedented sensitivity, with a measured
electrical NEP of $8 \times 10^{-22}$ W/$\sqrt{\mathrm{Hz}}$. This represents a
>100x advancement of the state-of-the-art, pushing TES detectors into the
regime where they may be employed the achieve to goals of even the most
ambitious large and cold future space instruments.
