According to Space.com, when engineering a sustainable space station, there must be a focus on closed loop efficiency, rocket propulsion and the sources of materials to be used in ongoing expansion projects. So far, these hurdles have been mitigated enough to get the International Space Station into orbit, but the technologies used are not currently advanced enough to support mass colonization of extraterrestrial habitats.
The article at Space.com argues that in order to support life in space, any station must be engineered to effectively travel between the station and Earth on an as-needed basis, recycle waste materials into usable substances autonomously,and move itself around in space relative to obstacles and gravity. As of 2013, chemical reactions were used to propel large rockets into space in the form of combustion of rocket fuels. These can be heavy, expensive and easily depleted from the spacecraft, so many scientists are focused on designing alternative fuels, like nuclear energy.
Furthermore, it is necessary to engineer high-efficiency recycling loops, which would allow for things like the conservation of water through filtering. Many scientists are interested in crafting loops that are 100 percent efficient, in which nothing is ever wasted, but the current loop used in the International Space Station is only 80 percent efficient.