The SAO/NASA Astrophysics Data SystemThe SAO/NASA Astrophysics Data System (ADS) is a Digital Library portal for researchers in Astronomy and Physics, operated by the Smithsonian Astrophysical Observatory (SAO) under a NASA grant. The ADS maintains three bibliographic databases containing more than 12.6 million records covering publications in Astronomy and Astrophysics, Physics, and the arXiv e-prints.