The third step is to make interplanetary spaceflight safe and affordable to everyone.
To do this we will need to build outpost space stations with local sources of propellant and interplanetary spacecraft with artificial gravity and plenty of shielding to protect the crew and passengers.
NASA wants to build an outpost space station at the Earth-Moon L-2 Lagrange point. Lagrange points, also known as libration points, are locations in space where the gravitational pull between two masses, in this case, the Earth and the Moon, are equal. Libration points are relatively easy to reach and easy to leave locations where a spacecraft or space station can be parked that requires very little station-keeping propellant.
NASA is also wanting to develop the technology to relocate small asteroids. This technology would allow small high water content asteroids to be moved to the outpost space station where they could be processed into drinking water, oxygen for breathing, propellant, metals for use with 3D printers, and shielding.
Doing this with existing launch vehicle would be very expensive. Doing this with a combination launch system that includes a Skyhook for boosting spacecraft to escape velocity would be very affordable.
Combine this outpost space station/propellant depot with the Nautilus-X spacecraft that NASA has proposed and regular on-going manned expeditions to Mars and the asteroids will become affordable and possible.
It will be the true beginning of manned interplanetary spaceflight.
