TOS: The Rings of Time, by Greg Cox
The U.S.S. Enterprise responds to an SOS from a vital dilithium-mining colony in the Klondike system. The colony is located on Skagway, a moon orbiting Klondike-6, a gas giant not unlike Saturn. For unknown reasons, the planet’s rings are coming apart, threatening the colony and its inhabitants. Kirk and his crew need to find a solution–fast. There are over three thousand colonists, including hundreds of families, on the endangered moon. That’s more than even the Enterprise can take on, and there are no other rescue ships or habitable planets anywhere in the vicinity. Meanwhile, a approaching comet that may be the source of the crisis threatening Skagway turns out to be a mysterious alien probe that seems to be on its last legs. Sensors indicate that the probe is incredibly old and running low on power. Suspecting that the probe may have something to do with the crisis threatening Skagway, Kirk has the probe beamed aboard the Enterprise. Scanning the artifact with his tricorder, Spock is unable to determine its origins, but detects no obvious threat. The surface of the probe is badly scorched and corroded, but Kirk can make out the remnants of alien hieroglyphics that look strangely similar to the ones on that alien obelisk he encountered on Miramanee’s planet years ago. Troubled by the memory, he reaches out to touch it…there’s a blinding flash…and Kirk suddenly finds himself floating in orbit above Saturn in our solar system, drifting in space wearing an old-fashioned NASA spacesuit. Disoriented, it takes him a few moments to realize that he is nowhere near the Klondike system and the Enterprise is nowhere in sight. Instead he glimpses an old-style spaceship of twenty-first century design several meters away. What just happened…?
Mirror Universe: Rise Like Lions, by David Mack