Thanks to President Obama’s directive, NASA has no current plans to return humans to the moon. However, as is common with space policy, this is subject to change, especially if a new president decides that it must be.
The question is, if a future president directs NASA to return to the moon or if somehow Congress forces the issue during the Obama administration, how would the space agency carry it out?
One option that has been studied by NASA concerns the use of deploying orbiting fuel depots to facilitate deep space voyages such as a return to the moon. The study, which was conducted in July, 2011 and has been updated since examined a number of scenarios for both an asteroid mission and a return to the moon, using the upcoming Falcon Heavy and the Delta IV Heavy. Each of the scenarios requires the launch of a fuel depot that could last in low Earth orbit for ten years.
Using primarily a Falcon Heavy, each lunar mission would require six launches, three with the Falcon Heavy delivering fuel to the depot, one Falcon Heavy launching a cryogenic propellant storage tank, one Falcon Heavy launching an Orion spacecraft and a lunar lander, and then a smaller, commercial launcher taking the crew up to dock with the Orion/Lander, transfer to it, and then, after it is fueled, take it to the moon.
Using the Delta IV heavy, five launches are required to fill the depot, one to launch the CPS, one to launch the Orion, one to launch the lunar lander, and one commercial flight to take the crew for a total of nine launches per lunar mission.
The study claims that each launch vehicle could support a lunar campaign with one mission every two years starting in 2024.
The main advantage of using fuel depots is that the great expense of building a heavy lift rocket such as the Space Launch System is avoided. The architecture also allows for greater participation of commercial and international partners. The crews could be send to the fueled Orion/Lunar Lander vehicle on any launch vehicle capable of taking a crew, from planned commercial spacecraft such as the Boeing CST-100 and the SpaceX Dragon to the Russian Soyuz and the Chinese Shenzhou.
The drawbacks to fuel depots were enumerated by former NASA Administrator Mike Griffin and former NASA Associated Administrator Scott Pace in a Space News oped. They cite doubt as to the cost estimates and reliability claims for various existing or soon to exist launch vehicles, the yet unsolved problem of storing cryogenic fuels in low Earth orbit, and the lack of expansion capability implied by foregoing heavy lift. Griffin and Pace suggests that fuel depots make sense in combination with heavy lift rockets such as the Space Launch system, using sources on the moob or an asteroid.