The space shuttle may be in the best shape of its life, but in the forward-looking world of space travel, it's a grizzled senior citizenwith mandatory retirement looming in 2010. The race is on to develop a replacement.Still on the drawing board, NASA's next-generation spacecraft will be called the "Crew" Exploration Vehicle, or CEV. It's expected to begin carrying astronauts to low-Earth orbit by 2014 and to the moon by 2020.Eleven aerospace-industry teams are vying for two NASA contracts to develop CEV concepts; the winners will be announced by early September.