Spectroscopy is at the heart of astrophysical inference.
Our understanding of the origin and evolution of the cosmos critically depends on our ability to make quantitative measurements of physical parameters such as the total mass, distribution, motions, temperatures, and composition of matter in the Universe. Detailed information on all of these properties can be gleaned from high-quality spectroscopic data. UV spectroscopy provides some of the most fundamental diagnostic data necessary for discerning the physical characteristics of planets, stars, galaxies, and interstellar and intergalactic matter. The UV offers access to spectral features that provide key diagnostic information that cannot be obtained at other wavelengths. COS will complement and extend the suite of HST instruments, ensuring that Hubble maintains a powerful UV spectroscopic capability from 2003 until the end of its mission.