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Russian Orbital Station




Origin of the international cis-lunar habitat

The first human crews on long-duration missions in the vicinity of the Moon will live and work in barrel-shaped modules less than five meters long. Even today's spartan quarters onboard the International Space Station might seem luxuriously spacious when compared to the obsessively compact design of the early cis-lunar habitats.

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Cis-lunar habitation module at a glance (as of Spring 2018):

Maximum spacecraft mass
approximately 10 tons
Body diameter
Internal: 4.2 meters; external: 4.5 meters
Full length
7.1 meters
Pressurized volume
76 cubic meters
Number of docking ports
4 (nominal); 6 (maximum possible with two modules)
Operational life span
15 years
Launch vehicle
Space Launch System, SLS
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As of beginning of 2017, the living quarters of the cis-lunar station, officially known as the Cis-lunar Transit Habitat, CTH, were to consist of two largely identical modules dubbed Common Habitat Element No. 1 and No. 2 or simply Hab-1 and Hab-2.

The European Space Agency, ESA, was expected to join forces with NASA to build these modules, based on Europe's experience in the highly successful Spacelab and Columbus programs, which provided Earth-orbiting modules for the Space Shuttle and the International Space Station, ISS. The Japanese aerospace agency, JAXA, also considers contributing its own habitation module or its major components.

The concept for the habitation modules was formulated by the end of 2016, after several years of consultations between the partners in the ISS project. Like the first component of the cis-lunar station, they were designed to fit into the NASA's super-heavy SLS rocket, packed below the Orion crew vehicle. At the time, the provisional flight manifest for the Orion/SLS system, called for the launch of habitation modules-1 and -2 during Exploration Missions-4 and -5 in 2024 and 2025, respectively.

In order to hitchhike into the deep space with the Orion, the mass of the module had to be kept under 10 tons. However, planners had to deduct one ton from this 10-ton limit for the Payload Adapter Fitting, which would connect the module to the rocket. Still more deductions had to be made for unexpected mass increases, leaving designers only 7,390 kilograms to work with, which is comparable to the mass of the Soyuz spacecraft.

The SLS rocket will first insert the Orion and the habitation module into the Earth's orbit along with a powerful space tug, which will fire in the direction of the Moon. Once in the trans-lunar trajectory, the crew will separate the Orion from the rocket, turn the ship around and dock it to the habitation module still attached to the empty rocket stage. The Orion/Hab stack will then separate from the rocket together.

Upon the arrival at the lunar orbit, the Orion will dock habitation modules one by one to the Power and Propulsion Bus, PPB, which would be delivered there first. The second habitation module will be rotated 90 degrees around its main axis relative to the first one, so that future visiting ships can berth at their side ports without bumping into each other with their solar arrays and antennas.

The Orion will remain docked at the outpost while the crew is tending the station, but once the expedition is over, the ship will carry the crew back to Earth, leaving the habitation module under the automated control of the PPB module.

Cis-lunar habitation module as of beginning of 2017:

Maximum spacecraft mass
7,390 kilograms
Body diameter
Internal: 4.2 meters; external: 4.5 meters
Full length
5.96 meters
Pressurized volume
63.7 cubic meters
Number of docking ports
4 (nominal); 6 (maximum possible with two modules)
Launch vehicle
Space Launch System, SLS


Design of the cis-lunar habitat

Each cylindrical habitation module will have four standard docking ports, known as IDSS, two of which will be located along the main axis of the "barrel" and the two so-called "radial" ports will be on their sides. Each docking port will have a hatch with a diameter of at least one meter to let an astronaut dressed in a spacesuit float through. Small windows on the hatches of docking ports will allow view outside and between modules. The four ports will be used to attach other modules, to receive crew vehicles from Earth and lunar landers returning from the Moon, including manned vehicles, as well as robotic spacecraft, such as soil carrying probes.

Six grapple fixtures will be spread around the exterior of each habitation module to enable the attachment of the robotic arm. A series of hand rails will be strategically spread around the module to help spacewalking astronauts.

In the preliminary architecture of the module, its thermal radiator panels could double as meteoroid shields. The areas which are not covered by radiators, would have specially designed aluminum shields bolted in. Fortunately, the cis-lunar space is practically pristine when it comes to artificial space junk, removing a major hazard faced by the Earth-orbiting space stations and thus allowing the developers to consider thinner walls.

Further lightening of the structure became possible thanks to a more straight-forward accommodation of the cis-lunar module on the SLS rocket comparing to strapping its ISS-based predecessors in the cargo bay of the Space Shuttle. As of 2016, the walls of the cis-lunar hab were expected to be around three millimeters (one eighth of an inch) thick in most places but reaching five millimeters around docking ports.

The barrel-shaped structure of the module is expected to be welded out of three aluminum rings with a diameter of 4.2 meters, which will be closed on both sides by cones with a length of 0.6 meters. A special system of accelerometers will likely be used to monitor the structural health of the module during its long life in deep space.

At the beginning of 2017, the developers also began considering the addition of a cupola-like window section, which could be delivered separately and docked at various available docking ports of the module over its life span.

Interior design

Each cis-lunar habitat will be equipped with state-of-the-art life-support, thermal control, communications, navigation, power supply and fire-safety systems. Inside, the module will have living quarters for the crew with private sleeping areas, which might use inflatable design to save space.

The galley/kitchen area will be as space efficient, likely featuring a deployable dining table, which will be lowered into position only for meal time. Other conveniences will include a fridge, which will likely double storing both food and scientific samples, and a highly efficient toilet, possibly hooked up to a recycling system.

A part of the first habitation module is expected to contain an exercise machine, which might require up to 10 hours of operation per day to accommodate four crew members.

The developers also consider including a work station, possibly featuring a crew-operated 3D printer.

Finally, the hab module will have some storage areas to hold food, water, clothing and all the other items typically found today on the International Space Station, ISS.

Like on the ISS, a big part of consumables for the crew will be re-supplied as needed with the help of dedicated cargo ships. The cargo ships will also be used to dispose of trash, whose projected volume onboard the small outpost presents a particular problem for designers.

Unfortunately, the standard racks with equipment used aboard the ISS today will not fit inside the cis-lunar habitat, first of all, because hatches between modules are expected to be too small. Also, there is a pressure on engineers to develop a more weight-efficient system. As a result, an entirely new secondary structure will have to be developed for the cis-lunar module to carry equipment, cable and pipe lines throughout its interior... Engineers are still debating whether to have the module fully configured for launch or have it outfitted during the flight.

The module's radiation protection system is yet to be detailed, but initial proposals include arranging flexible water bags around crew quarters. Some internal radiation shielding could also be introduced.

Cis-lunar station might lose its second habitation module


According to the new launch scenario released by the Multilateral Coordination Board, MCB, in March 2017, the launch of the first common element, then identified as a "habitat," was moved from Orion's EM-4 mission to the EM-3 flight. However the launch date according to the latest timeline turned out to be as before -- 2024. Moreover, due to its earlier position in the Orion's flight manifest, the EM-3 mission was expected to last 16 days with only five days left for orbiting the Moon, which was 10 days shorter than the EM-4 mission scenario. On the plus side, the shorter mission also means less food, water and other consumables taking up room aboard the ship, making it available for scientific instruments and other payloads, experts said.

Much more troublesome for the ISS partners was the complete absence of the second habitation module in the March 2017 scenario proposed by the MCB. Without the second hab, the station stood to lose a considerable part of its habitation space and logistical capabilities, including docking ports.

In his presentation of the MCB schedule, the head of NASA's human space flight program Bill Gerstenmaier stressed that the US agency would like and even want the second hab as long as one of the partners would be providing it. (It is unclear whether the "providing" also included the delivery of the module to the cis-lunar space, which could be a major challenge for the partners without the Orion/SLS mission allocated for the task.)

Not surprisingly, during the May 2017 meeting of the ISS partners in Montreal, Canada, the Japanese team officially requested to restore the second module in the architecture of the gateway.

Longer hab module proposed for the Deep-Space Gateway

To compensate for the elimination of the second habitation module, engineers considered various options to enlarge the remaining single module. Thankfully, the available volume within the cargo section of the SLS rocket, which was to carry the module, left some room to maneuver. Engineers considered several possible shapes for the new module concept.

By August 2017, it was decided to stretch the habitation module by almost a meter, making the barrel-shaped structure around five meters in length. This enlargment would naturally lead to some modifications in the cooling system, which in turn, could affect the power-supply system.

The engineers also proposed to add special tie rods on radial docking ports to stiffen the module's structure. They also wanted to add more passive connectors for scientific experiments.

While European and Japanese engineers continued parallel studies of their own habitation modules, NASA asked both partners to join their efforts. During a meeting in Noordwijk in August 2017, IECST recommended and program managers agreed to hold a face-to-face meeting around the following September to develop a single international habitation module concept with significant roles for ESA and JAXA.

One controversial issue around the hab was the Japanese closed-loop life support system known as ECLSS which stands for Environmental Control and Life Support System. The IECST agreed that ECLSS will provide clear advantages for missions exceeding 35 days, but the system was yet to be fully tested and validated on the ISS and could only be ready for practical use on the Deep Space Transport, DSP, spacecraft, expected to fly much later in the project.

2018: The partners prepare to enter a new phase in hab development

In April 2018, NASA, ESA and JAXA tentatively agreed that Japan would continue its work aimed at contributing the life-support system for the lunar outpost's main habitation module, which by that time was dubbed I-Hab, which probably a short for the international habitation module. NASA promised to contribute a power-supply system and a number of secondary items for the module, while the Canadian Space Agency, CSA, also looked at its potential contributions, most likely related to robotics.

In May 2018, the partners planned to begin two parallel efforts, known as Phase A and B1, further advancing the design of the habitation module, starting a development process aiming its launch for the middle of 2024. However, by that time, such a deadline seemed increasingly difficult to reach due to continuous delays on the political side of the LOP-G program.

A number of related small projects was also initiated to benefit the module's design, for example, the study of a new single-loop cooling system, as well as the work on the new-generation light-weight cold plates for mounting electronics and other light-weight structures.

The next major milestone in the module development, known as System Requirements Review, SRR, could be held in May or June 2019. As with other components of the outpost, the partners planned that these next phases of the habitation module design would be conducted under contracts awarded to the industry on a competitive basis.

To be continued


Functions of the Habitation module on the cis-lunar station:

Crew and equipment housing
Pressurized volume, sleeping quarters
Life-support system
Air ventilation, temperature, pressure and humidity control; removal of CO2 and other gases; other life-support functions; radiation protection
Handrails, photo and communications
Two axial and two radial IDSS docking ports (1 active; 3 passive)
Station integration Interfaces for power, data and intermodule ventilation systems
External payloads Interfaces for attachments of unpressurized payloads and robotic arm


Comparison of European and Japanese module concepts for the Deep-Space Gateway, DSG, as of August 2017:

4.2 meters
4.2 meters
Body length
4.9 meters
5.095 meters
Dry mass
7,793 kilograms
7,400 kilograms
Launch (total) mass
8,737 kilograms
Approximately 9,000 kilograms
Habitable volume
78.1 cubic meters
Approximately 76 cubic meters


New volume specifications for the hab module after its increase in length as of August 2017:

Total pressurized volume
76 cubic meters
Net habitable volume in primary aisle area
26 cubic meters
Volume for core systems
9 cubic meters
Volume reserved for outfitting during missions
23.5 cubic meters
Volume between bags and separation between cargo and the shell wall (interstitial space)
17.5 cubic meters

Next chapter: I-Hab module in 2019 (INSIDER CONTENT)


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The article and illustrations: Anatoly Zak; Last update: October 22, 2021

Page editor: Alain Chabot; Edits: October 6, 2017; May 3, 2018

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The Orion/Habitation module stack docks with the Power and Propulsion Bus, PPB, to form the human-tended outpost in the lunar orbit. Click to enlarge. Copyright © 2017 Anatoly Zak


The Orion crew vehicle undocks from the hab module at the end of Exploration Mission-4, leaving the cis-lunar outpost in automated mode. Click to enlarge. Copyright © 2017 Anatoly Zak


A European concept of the hab module for the cis-lunar station extended to more than five meters as of August 2017.


Click to enlarge. Credit: NASA


An interior of the habitation module for the Deep-Space Gateway as envisioned by engineers in 2017. Click to enlarge. Credit: NASA





The habitation module as of April 2018, known by that time as I-Hab. Credit: NASA


I-Hab module as of October 2018. Credit: NASA