Can Manned Spaceflight Save the Economy?

Barry asks: "Driving home last night I was listening to a particularly goofy AM talk station. Just before the syndicated UFO talk show 'Strange Days... Indeed' came on, the discussion turned to the Mars Rovers and George Bush's newfound love of space exploration. The interesting thought was that a large number of American political leaders were about to join Bush in endorsing a new manned space program because it would generate 'millions of jobs'. Given that manufacturing jobs are being shipped offshore, and high tech jobs are following, this almost made sense. A primarily unemployed population could mean big trouble. So I am wondering how many people were employed during the height of NASA's glory days, and what kind of economic impact would we expect if a similar program - a Mars mission for example - were launched today?"