NASA itself has drawn up design requirements for an Orbital Space Plane, which could carry crew to and from the station, and has funded the development of concepts for a robotic cargo vehicle. Together, the two craft could do everything the shuttle does, except deliver bulky expansion modules to the ISS. So what's taking so long? Politics get in the way. The shuttle program employs more than 18,000 people in congressional districts across the country. It takes this enormous workforce to service the tiny fleet -- only three shuttles remain after the demise of Challenger and Columbia -- because each vehicle has more than 2.5 million parts, many of which have to be replaced or refurbished after each flight.