Sometime around 1985, just after the Reagan administration had given the green light for NASA to build a space station in low-Earth orbit, in Building 15 at the Johnson Space Center, engineers were assembling a mockup of one of the station’s modules. The model, built specifically to fit into the payload bay of a Space Shuttle orbiter, looked a lot like the reusable Spacelab modules supplied by the European Space Agency, with lots of systems packed into the floor and equipment racks along the walls.

“The module was around 47 feet long,” said Gary Kitmacher, who was then an architectural manager for the Man-Systems Division of NASA’s Life Sciences Directorate, “and there were hatches on either end and four hatches around the outer part on one of the ends. We called that the Common Module. Most people thought that was the best way to build a module, and that’s what we started out with.”

“We learned some surprising things from the Skylab astronauts,” said Kitmacher, who is now manager of International Space Station Education and Communications. “One was that they were fine with being weightless, with floating around in zero g, but they really wanted a constant up-and-down orientation – and they wanted ‘down’ to be toward the Earth, by the way.”

As it turned out, the model being put together in Houston wasn’t quite the best NASA could do, but it was an improvement over the living/working quarters aboard Skylab, the only American space station to date. Skylab astronauts, including Gerald Carr, Bill Pogue, and Ed Gibson, had consulted on the design of habitable spaces for a new station. Skylab, whose Orbital Workshop compartment had been built inside a repurposed Saturn V rocket stage, was relatively roomy, at approximately 21 feet across and 48 feet long, a habitable volume of about 10,664 cubic feet. With the Multiple Docking Adapter and Airlock Module included, habitable volume grew to 12,417 cubic feet. Overall length with the Apollo Command/Service Module docked was a little over 118 feet. Inside, the workshop was divided into two stories, stacked along the tube’s longer axis. The ceiling of the “upper” deck opened into an airlock module, which connected to a docking adapter, which in turn connected to the vehicle the astronauts had used to travel to the station: the Apollo Command/Service Module (CSM).

“We learned some surprising things from the Skylab astronauts,” said Kitmacher, who is now manager of International Space Station Education and Communications. “One was that they were fine with being weightless, with floating around in zero g, but they really wanted a constant up-and-down orientation – and they wanted ‘down’ to be toward the Earth, by the way.” Many NASA engineers, who didn’t see the point of the concept of an up or down in space, were surprised. The Skylab workshop had an up-down orientation, but the docking adapter didn’t, and was completely disorienting to the astronauts. The CSM – whose interior, when docked at the station, was clearly visible – had a vertical orientation completely opposite that of the workshop.

The interior designs of these modules were guided by four specific principles: modularity, maintainability, reconfigurability, and accessibility.

Aerospace engineers in general tended to dismiss such concerns as irrelevant. Early space programs, such as Mercury and Gemini, had focused on the significant challenges of safety and survivability. But the long-term occupation of a space environment by a “microsociety” of people living and working together in confined quarters raised important psychosocial issues. NASA project leaders had identified “Man-Systems” as one of the nine primary systems of the new space station, on par with Electrical Power, Guidance, and others. Skylab astronauts had found the division of a module into vertical decks to be claustrophobic and visually confining. This feedback, and the fact that the station would consist of multiple modules in a configuration that required multiple exits from a single module, led to a preference for a longitudinal orientation for the module interiors, similar to that of the Spacelabs.

NASA designers worked closely with their counterparts at the European and Japanese space agencies, who would be building their own laboratories. The interior designs of these modules were guided by four specific principles: modularity, maintainability, reconfigurability, and accessibility. Interior hardware racks and utilities could be replaced easily, standardized so that they could be plugged into a slot in any module, and swung away for easy access to utility lines or the pressure hull.

Clustering ports and hatches at the end of a module required a significant portion of its interior volume to be devoted to the comings and goings of crewmembers, severely reducing available room for equipment or workspaces. They looked at the common modules and saw a lot of wasted internal space, taken up by all these hatches.

It took a considerable amount of architectural study to determine how to configure the interior space of the modules, and a surprising number of options were analyzed to maximize and optimize the volume devoted to habitation, storage, and utilities. The eventual winner was the “four stand-off” design: in cross-section, a square corridor running the length of the module, with four rows of standardized racks for storage and workstations. Between the runs of racks, hidden from view, four wedge-shaped tunnels allowed room for utilities – cabling, ventilation, fluid and gas lines, wiring and other electronics – that could be accessed by removing racks. It was an efficient, if austere, configuration, and one ISS crewmember, Canadian astronaut Chris Hadfield, later said the modules of the U.S. Orbital Segment created “an atmosphere similar to that of a hospital corridor.”

After lots of back-and-forth, and insistence by astronauts that direct observation with the eyes was a must for space operations, the Cupola project, canceled by the United States, was picked up by the Europeans, who built it under a barter agreement – the Cupola in exchange for transports of European crew on the Space Shuttle – and delivered it in 2010.

As programs were trimmed throughout the 1980s and 1990s, efficiency and function began to take precedence over habitability. The Man-Systems group was disbanded after Space Station Freedom became Space Station Alpha in the early 1990s, and the emphasis on the human-machine interface began to wane as the program budget was scaled back. The Habitation Module, one of the two larger U.S. modules planned for the station, was removed from the design, and crew accommodations such as sleeping quarters and exercise spaces were later added to the nodes.

One triumph of Man-Systems design was the Cupola. “A lot of our engineer types, and even a lot of our human/computer interaction people, basically said there is no reason for the astronaut staff to look out a window.” At the time, computers had little capacity for video, meaning the astronauts in the station would have been completely cut off from the outside world except for TV views. After lots of back-and-forth, and insistence by astronauts that direct observation with the eyes was a must for space operations, the Cupola project, canceled by the United States, was picked up by the Europeans, who built it under a barter agreement – the Cupola in exchange for transports of European crew on the Space Shuttle – and delivered it in 2010. It was the final building block of the U.S. Orbital Segment.

“It’s become very important,” Kitmacher said. “It’s the astronauts’ favorite place. Psychologically, I think it gives them a big morale boost.”

 

Structural Design: From the Power Tower to the ISS

 

In his 2016 book International Space Station: Architecture Beyond Earth, architect David Nixon details the earliest designs cooked up by a Johnson Space Center study group around 1983. Concepts such as the Delta, a cluster of modules connected to the bottom edge of a massive triangular prismatic truss structure, and the “Big T,” which featured modules along the edge of another rectangular mast structure that hung from a large crossbar, were audacious, ambitious – and completely impracticable, requiring multiple shuttle flights simply to lift and assemble their truss frameworks. A simpler truss structure was in order.

The design that began to take shape after Reagan announced U.S. commitment to the space station was known as the Power Tower, a 400-foot truss oriented vertically above the Earth, with modules clustered at the bottom, solar arrays branching out from the middle, and astronomy payloads at the top. Critics of the Power Tower pointed out that activity in the modules would send vibrations along the truss that could be amplified by its considerable length, disturbing sensitive microgravity experiments.

In 1985, a new design, the Dual Keel, moved the modules toward the station’s center of gravity. The Dual Keel was a rectangular truss, 310 feet by 150 feet, assembled in orbit from cubical 16- by 16-foot sections. Astronomy payloads were located on the rectangle’s upper edge, while Earth-sensing instruments occupied the lower. The modules were clustered at the center, on a long transverse truss that bisected the rectangle and bore solar arrays at either end.

The single keel station, also known as the “Revised Baseline Configuration,” is what appears in renderings of what Reagan dubbed Space Station Freedom in 1988. Space Station Freedom looks remarkably like today’s International Space Station, but the Revised Baseline needed further revision as the project, overweight and over budget, was passed on to the Clinton administration.

The ambitions of the Dual Keel were scaled back in the wake of the 1986 Space Shuttle Challenger disaster. NASA, in an effort to reduce demands on the Space Shuttle Program, erased the rectangular truss. The resulting single-keel design, a long transverse truss containing all the station’s elements, left the door open for adding more structures later.

The single keel station, also known as the “Revised Baseline Configuration,” is what appears in renderings of what Reagan dubbed Space Station Freedom in 1988. Space Station Freedom looks remarkably like today’s International Space Station, but the Revised Baseline needed further revision as the project, overweight and over budget, was passed on to the Clinton administration. The redesign was itself later tweaked, after Russia joined the project in 1993, to include Russian modules and a Soyuz escape vehicle. Two truss segments, P2 and S2, designated for their locations on the station’s port (left) and starboard (right) sides, had been planned as locations for rocket thrusters to reboost the station’s orbital altitude, but because the new Russian components provided that capability, the P2 and S2 trusses were removed from the design.

The design on the drawing board was now named the International Space Station.

 

Putting It All Together

 

The space station assembled in space from 1998 to 2011 is the largest human-made object ever to orbit the Earth, as long as a football field and weighing more than 900,000 pounds. Its solar arrays, with a surface area of more than 32,000 square feet, generate up to 80 kilowatts of electrical power. The station’s orbital altitude can range from about 200 to 250 nautical miles above Earth.

Assembling modules and components manufactured by multiple countries, often in different parts of the world, and launching them to match up perfectly while touching each other for the first time hundreds of miles above Earth presented ISS partner space agencies with an unprecedented logistical challenge. According to Kitmacher, the Russians had perfected this process in their years of building space stations. “Basically they have full mock-ups of their space station modules on the ground,” he said, “and before anything goes up, they bring everything into their modules, plug them in, and make sure everything works together.”

The pressurized modules of the U.S. segment relied on the use of Common Berthing Mechanisms (CBMs), roughly analogous to the Russian docking ports, developed by Boeing at the Marshall Space Flight Center. At locations where U.S. segment modules linked up with the Russian Segment, or with Russian vehicles, Pressurized Mating Adapters (PMAs) were attached to the Common Berthing Mechanisms to enable connection.

The Russian Zarya and Zvezda modules were selected to be the first functional modules sent into orbit because of their self-sufficiency: Each could maneuver and power itself using self-deployed solar arrays, and they would be relied upon to keep the rest of the ISS in orbit.

Another key difference between the assembly of the Russian and U.S. Segments was the ability of Russian modules, based on the design of self-sufficient Mir and earlier Salyut orbital station modules, to navigate, maneuver, and dock automatically. Modules of the U.S. segment were “berthed” – guided into place with the use of the Space Shuttle’s robotic arm, the Canadarm2, and the Mobile Servicing System, once it had been attached to the station.

The Russian Zarya and Zvezda modules were selected to be the first functional modules sent into orbit because of their self-sufficiency: Each could maneuver and power itself using self-deployed solar arrays, and they would be relied upon to keep the rest of the ISS in orbit.

In December 1998, three weeks after the Zarya was sent into orbit to become the first ISS module in space, the first two PMAs were launched with the American Unity Node 1 module, which would link the Russian and U.S. segments. Astronauts aboard the Space Shuttle Endeavour used the onboard robotic arm to grab onto Zarya and pull it close to the Unity’s location in the payload bay. After the two modules were mated, two astronauts performed spacewalks to connect electrical cabling and attach hardware to the exteriors.

The ISS was assembled in more than 40 missions between 1998 and 2011, and nearly all these missions went smoothly. The one glaring exception was the troublesome retraction and redeployment of the P6 truss segment’s solar array wings (SAWs). The truss piece, planned for the port-side end of the station, was actually the first of the segments to be sent up, in November 2000, to provide power to the station as it took shape. P6 was mounted temporarily at the center of the station, atop Unity and attached to the Z1 truss piece (a small structural segment, berthed for this very purpose, to Unity’s zenith, or upper, port).

The P6 SAWs later had to be retracted to make room for the deployment of solar arrays on the P4 and S4 truss segments, but the grounding of the Space Shuttle fleet, after the 2003 loss of the orbiter Columbia and its crew, had caused the arrays to be deployed for much longer than intended, a total of about six years. One of the accordion-like wings wouldn’t fold properly, a problem that required a total of eight spacewalks to overcome. In December 2006, astronaut Robert Curbeam, one of the crew aboard the shuttle Atlantis, unintentionally set a record for the most spacewalks in a single shuttle mission – four. A year later, on the assembly mission during which the P6 was finally removed from the Z1 truss and moved to its final home at the station’s port side end, one of its SAWs caused trouble again, developing a tear after unfolding to about 80 percent of its length. Two astronauts spent more than seven hours in a spacewalk to repair the array and unfold it to its full length.

The penultimate module delivery by the Space Shuttle to the ISS was achieved in May 2011, when Endeavour delivered the Alpha Magnetic Spectrometer (AMS-02) and ExPRESS Logistics Carrier to the station. While research had been conducted on the ISS since the first modules had been sent into space, the addition of AMS-02, a particle physics experiment module, marked the beginning of what’s known as the “utilization” phase – the era in which a fully mature, intact space station is operating at the height of its powers, enabling research that both improves life on Earth and informs the future of long-term space exploration.