Orbital refueling is done for decades; e.g. Progresses supplied Soviet space stations with fuel to maintain the orbit. What's trickier is orbital refueling of

Orbital refueling is done for decades; e.g. Progresses supplied Soviet space stations with fuel to maintain the orbit. What's trickier is orbital refueling of cryogenic liquids - LOX, liquid methane. It still looks easier than refueling with LH2, and ULA did a lot of work with that (search for "propellant depot"). Active cooling might be required; humanity had experience storing cryogenics in space for long time (e.g. Buran, which used LOX for RCS, was able to store it for a month on orbit), but we still might want a better solution (the one which actively turns vapors back into the liquid phase using onboard energy and radiators). Yet there is little which seems too complex in the refueling idea.