These two metal spheres are an example of space debris that did not burn up upon reentering the atmosphere but instead survived to land on Earth. Discovered at Mt. Stuart Station, New South Wales, Australia, in April and June 1963, they are propellant tanks that were used to store fuel for control mechanisms in a U.S. Agena D spacecraft that was launched in the fall of 1962. Agena Ds were one of several different types of Agena upper stages used atop many boosters, including the Thor and Atlas. The U.S. Air Force donated them to the NASM in 1965.
This object is not on display at the National Air and Space Museum. It is either on loan or in storage.