Buzz Aldrin has a plan to go to Mars.

Establishing private outposts in LEO [Low Earth Orbit] is just the first step in Aldrin’s plan for Mars colonization, which depends heavily on “cyclers” — spacecraft that move continuously between two cosmic destinations, efficiently delivering people and cargo back and forth…

Step two involves the international spaceflight community coming together to build cyclers that ply cislunar space, taking people on trips to the moon and back. Such spacecraft, and the activities they enable, would allow the construction of a crewed lunar base, where humanity could learn and test the techniques required for Mars colonization, such as how to manufacture propellant from local resources, Aldrin said…

Aldrin foresees these various cycler iterations enabling a crewed mission to a near-Earth asteroid by 2020 and a Venus flyby by 2024. If all goes well, the first future Mars settlers could launch in the early 2030s, he said.

So a sustainable Mars colony could be not an ambitious goal for the next 100 years, as Stephen Hawking just suggested, but 20 years or so out.