Heart of the Sun — Pamela Sargent

When an abandoned space habitat is found within a distant asteroid belt, the Starship Enterprise is sent to investigate. Captain Kirk and his crew discover an artificial world full of technological marvels -- and unexpected dangers. But wonder and curiosity give way to fear when the habitat's shifting orbit sends it on a collision course with an inhabited planet within the same solar system. Now Kirk and Spock must find a way to save the planet without destroying a treasure trove of alien science, and time is running out...