This is a cool little video clip and interview of Buzz Aldrin. During the Apollo 11 mission, the astronauts began to ask where the late stage rocket was.
The rocket was the one that separated from the shuttle a couple of days ago. They were asking about this position because they saw something outside which was tracking along with them in space. When the answer came back that their old rocket was 6000 miles away, they felt that they were looking at some other object.
Buzz Aldrin describes it as being ellipses from one perspective and L shaped from a different view. Several space missions from before reported seeing objects that were not explained. The Apollo 11 astronauts never mentioned the UFO over the radio because they did not want to cause a panic.
The UFO has never been identified or even officially acknowledged.