Complex sentence: Spock rushes to analyze the situation, and determines that the "Defiant's" phasing effect should recur within a few hours, when it does they may be able to retrieve the Captain, if he is still alive.
Split into simpler sentences: Spock rushes to analyze the situation, and determines that the "Defiant's" phasing effect should recur within a few hours. When it does they may be able to retrieve the Captain, if he is still alive.