Present Tense: The Janus Gate Book One of Three (Star Trek The Original Series)

After the Enterprise is hurtled three days into the past by the vast explosion of a dying world and into the proximity of an unknown planet, expeditions to the surface of the uninhabited world take on a perilous new meaning when Captain Kirk and Dr. McCoy mysteriously vanish and Spock, Scotty, and the rest of the crew must find them before their portal back to the present vanishes forever. Original.