The 50th Century is an era far into Earth's future.
Sometime in the 50th Century, a time capsule was found, left over from 1958. It contained relics typical of the 20th Century, but some people also wanted souvenirs of Superman's adventures. Therefore, they sent mental commands back through time, instructing him to leave space souvenirs in the time capsule. ("The Secret of the Space Souvenirs")