The Rings of Time — Greg Cox

2020 The U.S.S. Lewis & Clark, under the command of Colonel Shaun Christopher, sets off on humanity's first manned mission to Saturn. But the unexpected presence of a stowaway complicates the mission -- as does a startling encounter with an alien probe. But when Colonel Christopher attempts to capture the probe, he suddenly finds himself transported across time and space to a future era of space exploration. . . .

Stardate 7103.4 The U.S.S. Enterprise responds to an urgent distress call from a mining colony orbiting Klondike VI, a ringed gas giant not unlike Saturn. For unknown reasons, the planet's rings are coming apart, threatening the safety of the colony and its inhabitants. Searching for a way to avert the disaster, Captain James T. Kirk and his crew investigate a mysterious alien probe that has just entered the system. But when the probe is beamed aboard, Kirk abruptly finds himself floating in space above Saturn, wearing an old-fashioned NASA spacesuit, with the Enterprise nowhere in sight. . . .

Two missions. Two crews. And a time-twisting crisis that spans the centuries. . . .