The COS scientific investigation addresses questions of fundamental importance in astrophysics and cosmology that require the moderate spectral resolution and high throughput of COS, as well as four unique capabilities of HST: access to UV wavelengths, large collecting area, precise pointing stability, and excellent image quality. The study is organized into three broad categories, united by the theme of cosmic origins: (1) the origin of large-scale structure and the Intergalactic Medium (IGM); (2) the formation, evolution, and ages of galaxies; and (3) the origins of stellar and planetary systems.

This frontier science program critically depends on having the capability to obtain moderate resolution (R=20,000) spectroscopic observations of faint UV sources, such as distant quasars. Hence, the driving priority behind the COS instrument design is to optimize the spectrograph for maximum throughput.

COS will be, by a large factor, the most sensitive UV spectrograph ever flown aboard HST. Compared to modes of comparable spectral resolution on previous HST UV spectrographs, COS is ~8-10 times more sensitive in the wavelength range 1150 -1775 Å, and ~2 times more sensitive over the wavelength range 1750 -3200 Å. In the UV, maximizing sensitivity implies minimizing the number of optical components and leads to an inherently simple design. The unique capabilities of COS are due to a fundamentally different design approach and not from new technology. COS will build on the legacies of previous UV missions and instruments such as Copernicus, IUE, GHRS, FOS, STIS and FUSE, giving HST the greatest possible grasp of faint UV targets and ensuring that Hubble maintains a powerful UV spectroscopic capability from SM4 until the end of its mission.