The following is common knowledge amongst historians:
With the realization that the Transporter Dimension connects every point in the universe, the possibility of exploring distant planets orbiting distant stars became a much more realistic possibility than it had been before the Dimension Age. Of course, the fact that it was only possible to exit the Transporter Dimension at existing transporter pads meant humanity first needed to send transporter pads (aboard sub-light spacecraft) to each system it hoped to explore. The advantage was that the spacecraft carrying such transporter pads would not need to support life (even in cryo-stasis) for the decades or even centuries it might take to reach their destinations. Rather, control stations on Earth could track the progress of each spacecraft and transport astronauts only after each craft arrived at its destination. Such astronauts could then explore the nearby planets and return to Earth whenever they wished. The hope was that some of these transporter craft might someday serve as portals through which colonists from Earth might pass to establish thriving new pockets of humanity throughout the galaxy.
In the years prior to 2045, several transporter spacecraft were launched from Earth and began their journeys to distant stars. In most cases, the plan was to select crewmembers from future generations shortly before each craft’s anticipated arrival. In some cases, however, eager would-be explorers entered cyro-stasis pods set to awaken them upon the arrival of their designated transporter craft.
Unfortunately, the near-total collapse of human society following the Great Destruction put an end to most dreams of interstellar exploration. This left the various forgotten transporter craft continuing their lonely voyages to distant stars, with little chance that any human would ever use them.
[Opportunities for Adventure: PCs find the transporter coordinates for a spacecraft that left Earth before 2045 (i.e., more than 150 years ago). Alien creatures in a distant star system discover one of the transporter craft and begin using it to visit Earth.]