Complex sentence: An alternate plan, called "Plan B" by Tito, involves a mission 88 days longer beginning in 2021, would require a fly-by of Venus as well as a fly-by of Mars.
Split into simpler sentences: An alternate plan, called "Plan B" by Tito, involves a mission that would begin in 2021 but be 88 days longer in duration. It would require both a fly-by of Venus and a fly-by of Mars.