Buck Rogers in the 25th Century Wiki

During the period of time immediately after the Holocaust, groups of people begin to find ways to leave Earth and start life anew on other planets. Some remain in contact with "Terra," others do not, and some develop divergent cultures.

Ultimately all contact between Earth and these colonies is lost, although Earth and the colonies remain in touch from the early twentieth century through to the late 23rd century. ("Time of the Hawk," and "The Satyr")

At least one group of colonists sets up a viable community on Saturn. ("Journey to Oasis")

Also during this period of time, the Earth is looted by aliens. The frozen body of Hieronymus Fox is stolen. ("Cosmic Wiz Kid") This is likely the time of first contact between the humans of Earth and people from other worlds, and is not auspicious. This alien contact may have helped in the acceleration of human technology. (speculation)