Okay, here I go. Imagine these steps being taken in our quest to make a larger home for Earth based life:
1. A permanent manned presence at the lunar poles.
2. Mining capabilities at the lunar poles with solar powered mass accelerators to launch material into space.
3. Manufacturing capabilities in low Earth orbit (snuggly inside the van Allen radiation belts).
4. O’Neill space colonies at the Lagrange points of Earth (outside the van Allen belts but big enough to give you protection from radiation) manufacturing space-based solar power stations to beam power to Earth.
5. (Not necessarily in this order) A manned expedition to Mars.
6. The construction of a space elevator made of carbon nanotubes creating relatively cheap, clean access to geosynchronous orbit.
7. The creation of a fleet (5 to 10 maybe? 20? 30?) O’Neill space colonies placed in elliptical orbits around the sun with the perihelion (the closest spot on the orbit to the sun) being at the orbit of Earth and the aphelion (the spot farthest from the sun) being at the orbit of Mars. The more of these that there are, the better the chances of having one of them in the right position to take you to Mars or take you from Mars back to Earth. Short range shuttles could take passengers from the top of the space elevator to the space colony so that the space colony would never have to slow down or land on Earth or Mars.
8. A colony on Mars with it’s own space elevator.
9. A self-sustaining colony on Mars.
10. A terraformed Mars.
You would have a system in place where it would be relatively cheap, comfortable and safe to travel back and forth from Mars to Earth. It might take several months, but you’d have fresh food, gravity, protection from radiation. You’d have a multi-planet civilization. This means people who can communicate with one another, interact with one another, trade with one another.
Why try to terraform the moon? There isn’t enough gravity to hold an atmosphere and there’s far less water than there is on Mars AND you have a 28-day long ‘day’. There’s lots of stuff to do on the moon that will make life a lot richer for everyone, but living outside there is going to be a REAL challenge.
As for sending people on generation ships to take 10 to 50,000 year voyages to planets around other stars that will probably take some kind of terraforming when we get there but will never be in communication with us, to me that’s not creating a larger human family, that’s creating orphans.
Creating an interstellar civilization means making a whole lot more ‘what ifs’ including some fundamental breakthroughs in physics. But, who knows? I’m optimistic!