In 1961, President John F Kennedy famously announced his goal of landing a man on the moon and returning him safely to Earth before the decade was out. As we know, in July 1969, Neil Armstrong and Buzz Aldrin became the first people to walk on the moon, and were brought back to Earth safely, achieving JFK's goal.
At a time when most people hadn't even been on an aeroplane, landing on the moon would've felt unachievable and overwhelming. However, such a massive goal united people with a purpose; the story goes that even a cleaner mopping the floor at the space station said his job was to help put a man on the moon.
So, how did they make the goal achievable? They broke it down into milestones, with each one taking them closer and closer to achieving their ultimate goal.