Jim Gray explains that computational science has historically meant simulation and why there is an increasing role for analysis and mining of online scientific data. As a case in point, half of the world’s astronomy data is public. The astronomy community is putting all that data on the Internet so that the Internet becomes the world's best telescope: it has the whole sky, in many spectra, and in detail as good as the best 2-year-old telescopes. It is useable by all astronomers everywhere.   This is the vision of the virtual observatory — also called the World Wide Telescope (WWT). As one step along that path, Gray has been working with the Sloan Digital Sky Survey and CalTech to federate their data in web services on the Internet, and to make it easy to ask questions of the database. Gray explains the rationale for the WWT, and describes some the computer science challenges of publishing, federating, and mining scientific data. From the Series:CSE Colloquia - 2002