One of the things that makes it so challenging is that we're constructing the Station hundreds of miles above the surface of the Earth and we're doing it one piece at a time For the International Space Station we do not have the privilege of assuming the Space Station is on the ground before we take it up one piece at a time. So we have to be very clever about the testing that we do and the training that we do to make sure that each mission is successful, and that each piece and each mission goes just as it's planned.