One of the astronauts going out is Stephen Bowen, who was assigned to the job just last month. The original lead spacewalker, Timothy Kopra, was injured in a bicycle accident and pulled off the flight. Kopra will help direct the spacewalk from Mission Control. He's still on crutches.
This is the last flight for Discovery. After it returns to Earth next week, it will be retired and sent to the Smithsonian Institution. Only two other shuttle missions remain.