Watch a visual history of the ISS: The Station

Orbital outposts have come and gone over the years, but the most persistent by far is the International Space Station. Every other space station has been a relatively short-lived affair, lasting anywhere from a few months to a few years. But the ISS has been keeping humans alive and unraveling the mysteries of the universe for two straight decades — and it still has more to do before it's done. Private companies and other countries are starting to build their own orbital outposts, which means the ISS is probably the last space station that will ever be unambiguously referred to as The Space Station. It deserves this honorific. The ISS is an engineering marvel, a space age Notre Dame, a triumph of human ingenuity. It was also an utter pain in the ass to design.

Pulling together a space station is a Herculean task under the best of circumstances, but it’s like squeezing blood from a stone when three different presidential administrations, 15 countries, hundreds of American congressmen, thousands of engineers, and more than $150 billion of public funds are involved. It’s true Rome wasn’t built in a day, but it was probably built easier than the International Space Station. Depending on how you count, it took NASA 30 years to get the station it wanted. Even still, it pales in comparison to the agency’s original vision.

The dust kicked up on the moon by Buzz and Neil barely had time to settle before NASA was thinking about a permanent human presence in space. It wanted a 100-person outpost called Space Base, which would be a jump off point for sending crews and supplies to a base on the moon. It was an ambitious project — and an expensive one. It didn’t take NASA long to realize the cost of flinging stuff to orbit with expendable rockets would dwarf the cost of building the station itself. NASA needed something reusable. Something like a space shuttle.

“The biggest challenge of building any space station is getting the hardware up there into space,” says David Nixon, a British space architect and the author of International Space Station: Architecture Beyond Earth.

Before the shuttle, the world’s first orbital outposts — Russia’s Salyut stations and America’s Skylab — were launched in one piece on expendable rockets. Skylab was launched in 1973 on a Saturn V, the same rocket that carried Apollo astronauts to the moon. The rocket’s third stage was swapped out for a small cylindrical laboratory that doubled as a workshop and living space for up to three astronauts at a time. It also consisted of a number of smaller modules, like a solar observatory. It wasn’t fancy, but Skylab played a critical role in teaching NASA about the challenges of keeping humans alive in space for weeks on end.

America’s first space station was occupied by three different crews for a total of about six nonconsecutive months. The last mission to the space station was in 1974, and by 1979 Skylab’s orbit had decayed past the point of no return. It burned up as it re-entered the atmosphere that summer. But NASA had been bit by the space station bug. By the time the shuttle was ready — two years after Skylab’s demise — engineers at the agency were already thinking about the next step toward a permanent human presence in orbit. It was, after all, the shuttle’s killer app. In early 1982, NASA created a Space Station Task Force to study the problem and a few months later it doled out funding to eight top aerospace contractors who each received $1 million to come up with a viable space station design.

The contractors reported back to NASA with their designs in early 1983. Although each space station concept was unique, they were united by a few common design elements to meet NASA’s requirements. First, each station was designed to host up to 12 astronauts at a time, which was already a massive downsizing from NASA’s Apollo-era vision for Space Base. Most of the designs submitted also envisioned the station as a “spacedock” or “space harbor” that would be capable of repairing spacecraft and satellites in orbit.

But most notably, each of the space station designs submitted by the companies was modular. In other words, they were meant to be built in pieces, growing from a small kernel to a sprawling orbital complex. This feature was both pragmatic and necessary. The space shuttle could only boost relatively small portions of a station into orbit at a time, which meant that any space station would have to be built in space. However it also meant that the space station could be put to use before it was finished. This was an important consideration given that most designs were expected to take years to complete.

In 1984, about a year after the contractors submitted their designs, US President Ronald Reagan officially directed NASA to build a space station during his State of the Union address. He saw the space station, which he later christened Freedom, as a gateway to a robust space economy. “We can follow our dreams to distant stars by living and working in space for peaceful economic and scientific gain,” Reagan told the nation. “Just as the oceans opened up a new world for clipper ships and Yankee traders, space holds enormous potential for commerce today.”

Reagan’s State of the Union directive marked the beginning of a tumultuous period for NASA’s space station ambitions. Between 1984 and 1993, the official design for Freedom was subjected to seven major revisions as the agency grappled to balance budgets and engineering priorities. Within months of the president’s directive, NASA had a basic design for the station that didn’t really look anything like what its contractors had proposed just a year earlier. The new design was colloquially known as the “Power Tower.” It consisted of five modules—two laboratories, one logistics center, and two crew quarters—clustered around the end of a 400-foot long boom. The modules were at the bottom of the boom (that is, they were closest to Earth) and at the other end of the structure was a cluster of solar arrays and antennas.

The pendulum-like design of the Power Tower was meant to provide simultaneous views of the Earth, Sun, and space. This would aid in scientific experiments and reduce the amount of thruster firing required to keep the station in a stable orbit. But as designs progressed the Power Tower concept came under fire from scientists that wanted to use the space station for other fields of research. They argued that the habitable modules should be at the station’s center of gravity—not at the bottom—to limit the station’s movement. “The Power Tower had major structural problems,” says Nixon. “Putting the laboratory modules at the bottom of the structure was the worst possible place because of the oscillations and vibrations.” It was paramount that the design of the space station reflected its users’ needs and its users needed to keep the station’s movement to a minimum. So that meant NASA had to head back to the drawing board.

The agency released its new design for Freedom, known as the Revised Baseline Configuration, in 1987. In this architecture it’s possible to see the seeds of what would become the International Space Station. This time, the modules were clustered around the center of a single long boom that was flanked by arrays of solar panels. The modules also had international contributions, with Europe and Japan contributing laboratory components and Canada contributing a spacecraft servicing center.

The Revised Baseline Configuration was actually a compromise of a more ambitious design known as the “Dual Keel” that NASA had been exploring since 1985. Although this concept was also based around a long horizontal axis, it came with a large rectangular structure in the center—the dual keel—that could be used to host shuttle docking ports and other logistical modules. But this design was estimated to nearly double NASA’s original cost projections for a space station. So the dual keel element was abandoned to create the cheaper Revised Baseline Configuration — which had the option of adding the dual keel element at a later date.

Although the Revised Baseline Configuration ultimately served as the basis for the final design of the space station, NASA kept tweaking the design to meet continually evolving budget restraints and technology requirements. Generally speaking, each design iteration saw the space station lose capacity and caused launch schedules to slip. When Reagan announced the US space station program in 1984, the first modules were supposed to launch within a decade. But by 1993, NASA had spent $9 billion wrestling with space station designs, and still had little more than a stack of engineering studies to show for its effort.

Congress was fed up. That year, the entire space station program was saved from cancellation by a single vote in the US House of Representatives.

Still, the writing was on the wall: If the space station was ever going to become a reality, it was going to have to be a fundamentally international effort. The US clearly couldn’t muster the political will to do it on its own. So that year the Clinton administration officially 86’d plans for Freedom and announced that henceforth the US would pursue an international space station in collaboration with its allies. 

As far as the station’s design was concerned, the transition from Freedom to what would become the International Space Station was mostly a rhetorical maneuver. NASA had advocated for international collaboration on its space station from the start, and the European Space Agency and Japan had already agreed to contribute hardware to Freedom.

And now that the Cold War had come to a close, Russia was also invited to collaborate on this unprecedented project. “The Russians provided major support at a time when the program was just about to be canceled,” says Nixon. 

The final design of the International Space Station drew heavily on the Revised Baseline Configuration that had been fleshed out by NASA over the preceding six years, but with a simpler design. For example, the station’s main horizontal truss was delivered in prefabricated pieces rather than having astronauts assemble the truss in orbit. This simplified space station configuration consisted of a cluster of laboratory and habitation modules at the center of a long horizontal boom flanked by four solar arrays on each end. “The structure that emerged was something of a mongrel in the sense that it combined different ideas from different sources,” says Nixon. The entire station is the length of a football field and took more than 30 shuttle launches to complete. 

The US sent its first component for the space station — the Unity module that serves as the crew dining room — to orbit in 1998. Construction of the station officially ended 13 years later with the last flight of the space shuttle, which delivered a storage facility to the station.

The persistence of NASA and the world’s space agencies has paid off. Thanks to the ISS, there’s been a continuous human presence in space for 20 straight years, and the space station has provided a unique platform for groundbreaking science experiments. These days it hosts everything from a zero-G vegetable garden to a refrigerator that is the coldest place in the known universe. It’s also beginning to fulfill Reagan’s vision as a springboard for extraterrestrial commerce. Companies have used the ISS to study everything from “organs on a chip” to 3D printing to baking cookies, and some are even using it as a starting point for building space stations of their own. 

It’s an incredible legacy considering the decades of wrestling with international partners, slippery budgets, and combative presidential administrations.

It may not have been cheap or fast to design The Space Station — but no one ever said space was easy.