Apollo 9 doesn’t get a whole lot of attention in various space histories since the mission never left Earth orbit. Still, it was an important stepping stone on Apollo’s way to the moon.
The Apollo 9 mission was the first full-up test of all the hardware that would be used at the moon in later missions — including the lunar module. Over the course of ten days, the Apollo 9 crew docked the command & service modules with the lunar module, then undocked, and redocked repeatedly in order to give all elements of the system a thorough workout.
Somewhat ironically, the Apollo 9 mission was not the end of this capsule’s travels. Originally, this module was on display at the Michigan Space and Science Center in Jackson Michigan. But in April of 2004, the center closed — and the command module was moved to its current berth in San Diego.