Shadow Planet(Quest for Tomorrow #5) by William Shatner

The scum of the galaxy are the last hope of humankind . . .Aboard the stolen, renamed starship Endeavor, the Stone Cowboys rocket across the heavens toward a confrontation they cannot avoid . . . and, most likely, will not survive. But first, Jim Endicott must mold the street-hardened gang of thugs and hoodlums into a disciplined commando unit. Once harboring dreams of joining the Space Academy, Endicott has already altered one past using the astonishing powers of Omega. And now the untried captain and his misfit crew must take on a powerful alien race devoted to the obliteration of the human "cancer." With tension, unrest, and mutiny brewing dangerously all around him, Endicott faces the deadliest challenge he has ever known. Because the cutthroats riding the Endeavor into the enemy's turf for a war to the death are the only champions courageous -- and foolhardy -- enough for the mission. And they have nothing in the universe left to lose . . .