John Kerry brought two astronauts with him to campaign at the Kennedy Space Center. He strolled among the rocket relics. And he recalled a day when those rockets were the tools a dedicated army of Americans used to do what then seemed impossible. But in the heart of a community where more than 20,000 people work on space programs, the Democrats’ candidate for president met with a roomful of voters without commenting on President Bush’s proposal to send astronauts to the moon and Mars or offering a specific vision of his own for exploring space.