This paper presents a novel concept for a compact low-cost Earth sensor based on imaging atmospheric oxygen emission at 762nm using arrays of single photon avalanche diodes (SPADs). In both nighttime and daytime there is continuous emission at 762nm due to oxygen recombination. The SPADs high sensitivity enables the ES to operate at night and day, over a wide temperature range, with a very compact optical system (e.g. aperture of 10mm, focal length of 14mm) and no scanning elements. A modular and low-cost instrument design is described with uses the same wavelength band, the same detector technology, the same optics, the same power and data interfaces and similar algorithms for LEO and GEO applications. Using the visible band rather than the LWIR band offers several advantages including much lower cost detectors, no need for cooling, a more relaxed thermal design, simpler optics, higher sensitivity and hence lower cost.