IEEE Spectrum: The agency is acutely aware of this, which is why it's been trying to get commercial carriers to handle deliveries of (now) supplies and (soon) astronauts to the ISS. By Evan Ackerman
'The next step is for private companies to take over space station construction for (soon) Earth orbit and (eventually) deep space. To that end, NASA has selected six partner companies to develop full-sized ground prototypes and concepts for deep space habitats, with the eventual goal of deploying habitats near the moon as a stepping stone to Mars.
'Five of the partners, including Bigelow Aerospace, Boeing, Lockheed Martin, Orbital ATK, and Sierra Nevada will be designing habitats that are built on Earth and launched into space on rockets. It makes sense to do this, because it's how habitats have always been sent into space. The sixth partner, NanoRacks, is teaming up with Space Systems Loral and United Launch Alliance to try something completely different: taking empty fuel tanks from the upper stages of rockets and turning them into space habitats on-orbit.
'The hydrogen fuel tank on a Centaur upper stage has a diameter of over 4 meters, and an interior volume of 54 cubic meters. By way of comparison, the inflatable BEAM module that arrived at the ISS earlier this year has an interior volume of 16 cubic meters. Centaur's fuel tank is pressurized, rugged, and most importantly, already in space for free, and NanoRacks wants to leverage that to create inexpensive space habitats for humans.'