The rear end of the space shuttle, Enterprise, at the Udvar Hazy Center at Dulles. Originally this was slated to be named the Constitution, but a massive write campaign clamored for the name Enterprise and NASA gave in.
This shuttle was built for testing - never had engines or a functional heat shield. The plan was to retrofit it for flight, but due to changes in the design that never happened.
Someday this shuttle will be replaced with the Discovery and the Enterprise will be available to loan to other museums.
Isn't the Smithsonian a terrific institution?