When you first start playing, you'll notice how free you are to create your own space missions. You can design and build your own rockets, choose your crew, and plan out every detail of your journey. It's pretty intimidating at first, but in a good way - you'll feel like you're actually running a space program.
As you play, you'll be switching between designing your rockets, managing your resources, and actually flying your missions. It's a great feeling, knowing that you're in control of everything and that your decisions will affect the outcome of your missions.