Space shuttle OV-098 Pathfinder on display at the US Space & Rocket Center in Huntsville, Alabama. Pathfinder was used as a structural fit test article and for training. After it was retired, it was rebuilt to look more like a space shuttle on the interior, and sent to Japan for an expo. Once back in the states, NASA put Pathfinder on display along with mock-ups of the external tank and solid rocket boosters. Note that OV-098 is an honorary designation given to Pathfinder after it was retired. The external tank displayed with Pathfinder was originally part of MPTA-098, and was used in the development and testing of the SSMEs (space shuttle main engines).