As universities are saying goodbye to the graduating classes of 2004, the National Aeronautics and Space Administration is welcoming a new freshman class of astronauts. The 11 trainees are joining the astronaut corps at a time when the space program faces its greatest challenges and perils since the moon shots. The group includes four who already work at the Johnson Space Center in Houston. Two of the 11 are women, and three are schoolteachers who will train as astronaut educators.