text:  NASA has begun testing on Earth a key part of Boeing's newest spacecraft. As NPR's Jeff Brumfield reports, the testing was ordered after several key parts of the spacecraft failed while the craft was in flights. As it went to dock with the International Space Station, Boeing's new Starliner spacecraft ran into trouble. Five maneuvering thrusters suddenly stopped working. Engineers eventually got four of the five back up and running, but they still don't know what went wrong.