The International Space Station (ISS) may never again be visited by a space plane.
NASA's space shuttle orbiters were instrumental in building the ISS, and in keeping it operational for its first decade; these reusable spacecraft ferried astronauts to and from the orbiting outpost until the fleet was fully retired in 2011. Five years later, the agency opened the door to more space plane meetups, signing a cargo deal with Sierra Space, the Colorado-based company behind the robotic Dream Chaser vehicle.
