The first successful Earth-Saturn probe, commanded by Colonel Shaun Geoffrey Christopher, whose importance to history obliges Kirk to return the colonel's father to 1969.
The Earth-Saturn probe is mentioned in the original series episode Tomorrow Is Yesterday. After the Enterprise is thrown back to 1969 and accidentally destroys the jet of Air Force Captain John Christopher, Kirk beams the pilot aboard.
Spock at first reports that Christopher made no notable contribution to history, so that he could safely remain in the future. He then corrects himself: the captain's son, Shaun Geoffrey Christopher, not yet born, will command the first successful Earth-Saturn probe.
That discovery means John Christopher has to be returned to Earth with no memory of the starship. The date of the mission is not given in the episode. The flight is otherwise a single line of future history.