[giantg2]

I believe they already have the propellant transfer worked out. There was a paper a few years back looking into Musk's general plan. They already had some mechanism for refueling. If I remember correctly, the biggest questions were around lifesupport systems with their efficiency and heat shields. Which were technically still solvable with current technologies and slightly different approaches.

From my perspective, the thing they're mostly waiting for is additional tech to make everything easier and lower risk, or for an aggressive administration that can push it and fund it.