This article describes the design and performance of a 480 GHz flat lens, 150 microns thick, 124 mm in diameter and weighing 3 grams. The lens, fabricated with alternating layers of spin-coated polymide and patterned aluminum, has nearly diffraction-limited directivity with a measured loss of 2.5 dB. The lens has been designed close to the 557 GHz ground-state transition line of ortho-water for envisioned SmallSat and CubeSat applications to observe ground-state transitions of water and other interesting molecular lines from space.