Welcome to "Risk and Exploration-Earth, Sea, and the Stars." Today's session is entitled "Why We Explore," but I'm hoping that, mostly, we can make it a dialogue, up close and personal. I'm John Grunsfeld. I'm the NASA chief scientist and an astronaut.I think we have started getting into the discussions on risk and exploration, into some of the thorny questions about how do we make decisions. How do we use our judgment? How do we, as institutional managers of a public institution, make decisions on behalf of the American people, and with oversight of the Congress, that can stand the test of time, without being so risk averse that we don't do anything interesting?There's a couple of things I'd like to show this morning that are personal, that are professional as chief scientist, and then, representing the Agency, and then, looking forward. I think we'd be remiss in all of this discussion if we avoided the topic of why we're not sending a Space Shuttle back to the Hubble to service it. So I'll address that in a second.