Complex sentence: Captain Kirk remembers the "Franklin" had been declared lost by the then newly formed Starfleet and since he finds it at a vast distance from Earth, he theorizes that it ended up on the planet via a wormhole.
Split into simpler sentences: Captain Kirk remembers the "Franklin" had been declared lost by the then newly formed Starfleet. Since he finds it at a vast distance from Earth, he theorizes that it ended up on the planet via a wormhole.