He's probably implying mining or synthesizing fuel on smaller gravity wells like Mars. Being able to synthesize fuel for the return trip could massively cut bac

He's probably implying mining or synthesizing fuel on smaller gravity wells like Mars. Being able to synthesize fuel for the return trip could massively cut back on initial launch weight. Energizing propellant with a microwave beam (https://en.wikipedia.org/wiki/Beam-powered_propulsion https://en.wikipedia.org/wiki/Beam-powered_propulsion) has been hypothesized, while a solar sail is much more realistic with modern technology.