It would take the station a week to make each revolution around the Moon in such an orbit. Although the NRHO is a primary location for the base, the outpost would still be able to maneuver to other destinations in the future.

The partners are also said to be on track to finalize their common technical standards for the station as early as April. The critical agreement will make sure that all the hardware and technology needed in the program, such as rendezvous and docking systems, life support, communications, power supply and thermal control gear will work seamlessly and safely for years to come.

According to the latest design, the station includes a pair of habitation modules built by Europe and Japan, a U.S.-built power and propulsion module, a Russian airlock module and the 8.5-meter Canadian robotic arm. Based on the experience obtained during the ISS program, the life-support system of the cis-lunar station will eventually feature a truly "closed-loop" technology, essentially recycling all its resources, such as waste water and oxygen.

Engineers are also considering adding a 360-degree "glass" deck on the station with multiple windows, enabling future astronauts to enjoy lunar vistas, much like today’s ISS crews observe the Earth through the panoramic windows of the cupola module. Depending on the mission, the new-generation cupola could be bolted to different docking ports of the station.

NASA’s Orion spacecraft, launching on a giant SLS rocket, would conduct annual trips to the station, delivering crews and most of the outpost’s components. In the latest move agreed to in Tsukuba, Russia will also evaluate the possibility of using its new-generation Angara-5 rocket to carry its airlock module to the station.

According to current plans, the cis-lunar outpost would be under construction and operation for much of the 2020s. Still, this scenario is considered only as the first step in enabling human missions further into space, such as expeditions to asteroids, the moons of Mars and ultimately the Martian surface during the 2030s.

Despite big progress on the engineering front, the project faces an "elephant in the room" in terms of political support. The new White House administration has yet to lay out its vision for NASA’s human spaceflight program. International partners are also not on the same page in terms of the project’s ultimate goal. Russia, for instance, wants to focus the station on the exploration of the Moon rather than its use as a test bed for a mission to Mars as insisted by NASA. During the latest meeting in Tsukuba, Russian officials reportedly proposed an alternative design of the outpost aimed at the Moon. Roskosmos promised to analyze whether keeping the station in a low lunar orbit would be feasible.

To address potential disagreements on where to go with the cis-lunar habitat, NASA promised to study how to adapt the outpost for both tasks. One solution would be keeping some of the infrastructure near the Moon permanently, while other pieces could later be detached in order to go to Mars or other deep-space destinations.

Engineers are also working to ensure an open architecture design for the new outpost, where new modules or components could be "plugged-in" into the station, in case new international or commercial partners join the project.

In preparation for possible political and public relations battles ahead, NASA proposed to name the cis-lunar station a spaceport, while the future Mars-bound part of the project would be dubbed a spaceship. Interestingly, the spaceport concept in lunar orbit appears in early Russian studies of the project circa 2005.

Although all the key components of the cis-lunar station are to be delivered by NASA’s giant SLS rocket, there is an open question on cargo missions, which will be needed to support the crews during progressively longer missions. Although no decision on cargo delivery has been made yet, it is possible to speculate the agencies might want to outsource logistical operations to industrial contractors, which would provide their own launch vehicles and spacecraft. In that scenario, U.S. companies like SpaceX and Orbital ATK could have big advantages in any bidding processes, thanks to their experience in delivering cargo to the ISS on the Dragon and the Cygnus cargo ships, respectively.

Just two weeks after the meeting in Tsukuba, SpaceX announced its intention to send a pair of tourists on a mission around the Moon onboard its Crew Dragon spacecraft, launched by the Falcon Heavy rocket. Although the company’s deadline for the cis-lunar mission at the end of 2018 is not considered realistic, the rocket and spacecraft could mature in time to support a cis-lunar outpost. The company’s Falcon Heavy can deliver more than half of the payload carried by NASA’s SLS rocket.

Assembly of the station is currently proposed to begin with the third flight of Orion around 2023. The first logistics flight for the cis-lunar outpost might be required between 2024 and 2025, sources said.

For more historic and technical background on the cis-lunar project, visit Russianspaceweb.com.

Anatoly Zak is the publisher of RussianSpaceWeb.com and the author of Russia in Space: the Past Explained, the Future Explored.