THE INTERNATIONAL SPACE STATION / МЕЖДУНАРО́ДНАЯ КОСМИ́ЧЕСКАЯ СТА́НЦИЯ — Russia/USA/EU/Japan, 1998

It’s the most expensive moving vessel ever made. It’s also the most expensive building ever built, and quite probably the most expensive artificial object of any kind in history, unless you count something like an entire national highway system as a single construction. It has been occupied continually now for twenty years. And it’s becoming increasingly commercialized. It’s the only one of its kind, but it won’t be for much longer... building another one can now be done far cheaper.

Overall, one end of the station is Russian and the other is American. They were originally going to be two rival stations, Mir 2 and Space Station Freedom, but after the fall of the Soviet Union they decided to stick the two together so they could cooperate. Even now, it may be possible for the two to separate and be independent, and with recent political tensions, the idea has been brought up. (One thing that would make this problematic is that though the Zarya module is Russian built, it’s legally owned by the United States, which paid most of its construction costs.) To spread the cost around, additional help was recruited from Europe and Japan. (Some nominally American parts were built in Italy.) The Chinese were pointedly not invited. Fresh memories of the Tiananmen Square massacre may have been a factor in the decision. Since then, concerns about sharing our space technology have kept the Chinese excluded even as visitors.

When looking at the station as it is today, it's easy to just see a tangled jumble. But the layout can be understood by starting with the core — the backbone of the structure. This consists (currently) of a stack of five pressurized modules. Each module is sized to make a full load for a shuttle or a Proton or whatever other craft might bring it up, weighing up to twenty tons. It took about two dozen launches to build the station up to its present size.

To navigate the station, it helps if we have a clear sense of direction when moving about. Fortunately, the station mostly keeps a quite consistent orientation relative to the Earth, with one end pointed in the direction of orbital motion and one side faced toward the ground (except sometimes when docking or maneuvering, when it may angle itself differently, as in the image above, where its underside is facing forward). This allows the people on it to designate certain directions as forward and aft, port and starboard, above and below, like on a traditional ship. (But for the latter, they prefer the terms zenith and nadir.) Or, given its shape, one might be more tempted to compare it to an airplane instead of a ship, with the solar panels and so on taking the role of wings. The five main modules are fairly comparable in size and shape to an airliner’s fuselage... but this analogy breaks down once you start looking at the side modules.

Generally speaking, these modules have a round cross section on the outside and a square one on the inside. Each wall is tall enough for someone to “stand” in front of while working with equipment mounted there, and having it flat instead of concave makes this easier. Cubbies such as sleeping nooks and bathrooms have a flat door and a concave back wall. Installed equipment is designed to fit into standardized slots with round backs and flat fronts, each about the size of a fridge.

central modules

Let’s list off the five main modules, going forward from the aft end:

• “Звезда́ / Zvezda” (Star): this is the main area for the Russian section, where people exercise and sleep, and do some lab work. NASA calls it a Service Module. Its back end has small rocket engines that can be used to adjust the station’s orbit, or it can dock a spacecraft there. This module is considered the safest place to go on the station in case of an emergency — the point to retreat to if anything goes wrong. It is the location of the primary life support systems — those on the American side are considered supplemental only, at least on paper. It has its own solar panels and reaction control thrusters, and a few windows. It only sleeps two but these quarters have portholes, so one of them is usually given to the station commander. The front end, which joins it to the next module, is a junction which has two more docking ports above and below. This ball-shaped junction is dotted with windows for use during docking, EVAs, and so on.
• “Заря́ / Zarya” (Dawn): the oldest part of the station — the first module sent into orbit. Originally called the Functional Cargo Block. It has its own solar panels but they are currently retracted. As its functions have been obsoleted by other modules it has fallen into relative disuse and nowadays mainly just acts as a storage area. It has reaction control thrusters.
• “Unity”: the first American module, attached to Zarya via the same sort of mating adapter that spacecraft such as a Shuttle or a Dragon 2 would dock to on the outside. This is an important crossroads. It is also known simply as Node 1. This acts as the galley — the main area where food is stored and eaten. A growing number of add-ons go off the sides of this. It has an available berthing port on the bottom. It’s a stubby little thing, just 5.5 meters long. It was built by Boeing, as was:
• “Destiny”: the second largest module at 8.5 by 4.3 meters, this is the main working laboratory of the station. It has one window of telescope-quality glass for precise observations of the Earth. It also has a stationary bicycle, which is an odd fit with all the experiments. The gigantic crosswise truss which holds the main solar panels and heat radiators is attached to the top of this section. All kinds of nonpressurized equipment goes up there, from the batteries that supply power at night to the gyroscopes which keep the station right side up. (They sometimes use these to turn the whole station in order to make docking easier.) When it’s time to “desaturate” the gyros, they have to use the Russian thrusters, or those of an attached spacecraft, as the American section has none. If ever needed, thrusters could be put onto the truss, but these bits were never launched. Some old parts of the truss were originally attached to Unity, which still has a truss midsection known as Z1 atop it.
• “Harmony”: the forwardmost American module. It’s similar to Unity but 7.2 meters long, and is alternately called Node 2. This has sleeping quarters for four astronauts, which lack windows but have better sound insulation than those in Zvezda. It also has side branches, plus it has docking ports on the front and top, and a berthing connector available on the bottom. The two docking ports on the American end are an update of the APAS style used on the Russian end... but despite the common heritage and relatively minor differences, the ports at the two ends of the station are no longer compatible, as the Russians have not updated their legacy Soyuz and Progress spacecraft, due to the weight it would add. This port has a round opening 0.8 meters in diameter. The American modules connect to each other using a 1.3 meter squarish port called the Common Berthing Mechanism, and many cargo craft such as the Cygnus get berthed to this port directly — an operation that requires the Canadarm. For passenger craft that dock themselves actively, the round docking ports are actually external adapters which are fitted onto the berthing ports with the Canadarm, and get moved from time to time. Nowadays Harmony has both of these adapters, one of which used to be Unity’s. (The adapter connecting Unity to Zarya is the same except not updated. It even includes the diagonal offset that was designed to maximize access to the Space Shuttle's cargo bay.) Though similar to Boeing’s Unity, this was built in Italy by Thales Alenia Space.

That’s a total of six sleeping spaces in the core modules. When seven or eight people are up there, someone — often the commander — uses a docked spacecraft as a bedroom, which offers more room than the scant 2.1 cubic meters of a normal cabin, but less privacy. Space is tightest during a crew change, when for as long as a week there might be up to eleven people. At crowded times, schedules for the exercise machines need to be tightly managed. This gear stays busy because in zero gee, it takes like two hours a day of workouts to stay healthy.

So if you think of the station as a small town, these five modules constitute its main street. It’s a continuous tube that allows you to float from one end of the station to the other in a nearly straight line — the main traffic artery. Now let’s look at some of the side streets.

side modules

The longest cross street is at the front, branching off of Harmony. Up here you find:

• “Columbus”: a smallish laboratory added by the European Space Agency, on the starboard side. A lot of the biological and medical experiments are done here, including little hydroponic gardens so the astronauts can have a bit of fresh produce to eat. This kind of gardening will be crucial for future space explorers and colonists, so they have been working hard at iteratively improving this process for years now. They say the food you miss the most after six months in space is salad. It was built by Thales Alenia. Recently a platform called Bartolomeo was added to the outside. In 2021 they added quarters here for a seventh crew member to sleep in, due to NASA flights now bringing four at a time instead of three, but this is temporary.
• “きぼう / Kibō” (Hope): a laboratory from JAXA (the Japanese space agency) on the port side. NASA calls in the Japanese Experiment Module. It has a small additional module stuck on the top side which is basically just for cargo storage, and a large protruding array of external equipment, known as the Terrace, plus a medium sized robot arm to operate the outside stuff with. It even has a small airlock at that end, so stuff can be moved between the Terrace and the interior. This is the second newest major module, and also the biggest — 11.2 by 4.4 meters, which makes it about double the size of Columbus even without the storage add-on. The Terrace sticks out an additional 5.6 meters. It took three shuttle flights to build this section.

Back in the middle, branching off of Unity, we have the most complicated part:

• “Quest”: the main airlock, on the starboard side, where space suits are kept ready for outside work. The process of getting in and out is quite prolonged, because the astronauts have to move from normal air at sea level pressure to pure oxygen at low pressure, or it would be very difficult to bend the joints of the suits. Made by Boeing.
• “Tranquility”: a module on the port side also known as Node 3. They asked the public to vote on what to call it, and all the Firefly fans made “Serenity” the winner, but they weren’t willing to go there. This is a smallish module (6.7 meters) but a very popular one, because its underside has the famous Cupola — a little room with big windows, which is mainly used just for gazing at the beauty of the Earth, though its official justification is for monitoring spacewalks and Canadarm operations. Tranquility also has a toilet, unlike Unity or Harmony. (Until 2021 the only other toilet was in Zvezda, though in a pinch you could use one on a docked spacecraft.) The COLBERT treadmill is also there, after previously residing in Harmony, and so is the ARED “weight”-lifting device. Tranquility, like the other two Nodes, has berthing ports on it facing in all six directions, and most are occupied. (The top port is unused because it’s crowded by the truss.) Like Harmony, this was built by Thales Alenia, as was the Cupola.
• “Leonardo”: this currently sticks forward from Tranquility, parallel to Destiny, and is also known as the Permanent Multi-Purpose Module. It’s fairly large, but it doesn’t get much use anymore except as storage. Before this became permanent in 2011, it used to ride up and down in shuttle flights, and be berthed in any convenient spot (often the bottom of Unity, where Cygnus cargo carriers sometimes go nowadays), along with two sibling modules named Raffaello and Donatello. (No Michelangelo yet, and definitely no Splinter.) A lot of the larger and bulkier interior fittings rode up in these during construction. They were made in Italy by Thales Alenia Space. They also built the ATV cargo craft, and supply the pressurized section of Northrop’s Cygnus cargo craft. Before installing this permanently, they had to thicken its radiation shielding.
• “BEAM”: the Bigelow Expandable Activity Module, a test of inflatable technology, sticks out the back of Tranquility. Apparently this soft construction should actually offer better protection against meteorites and radiation than a traditional metal can does. But measurements have not shown any radiation advantage over the other modules. It’s small, but they were planning to make much bigger ones if it was successful. Such plans may be shelved as Bigelow Aerospace is now inactive, but other companies are at work on similar technology. At present the module is just used as a small storage closet, and the hatch is kept closed, due to its unproven nature. (The greatest risk of a blowout, however, might be in Kibō and Columbus, which are where a broadside of space junk is most likely to hit. They’ve checked the front of Columbus and found hundreds of little pockmarks there already.) BEAM may be discarded into the atmosphere in the next year or two.
• “Bishop”: a commercially owned airlock, recently installed on the outermost port of Tranquility, where in the past they had mounted a docking adapter. It was made by a company called Nanoracks, with some parts from Boeing and Thales Alenia, and rode up in a Dragon trunk. Apparently the small airlock on the Kibō module was a bottleneck, so they made a new one, which can handle larger stuff. This is not for space suits, but just for moving smaller experiments between the inside and outside. It is the smallest module on the station, barely larger than the Cupola.

The Russian modules each have some attachments, most of which are much smaller than the main modules, but still sizable compared to small fry like BEAM or Bishop. And unlike the international section, where the side modules protrude to port and starboard, the Russian ones are vertical, sticking out above and below.

• “Рассве́т / Rassvet” (First Light), also known as Mini-Research Module 1, sticks down from the underside of Zarya’s junction ball. It was originally supposed to be a full sized module, and was replaced with this little version (6.0 by 2.35 meters) due to budget cuts. The bottom end is a docking port for Soyuz and Progress capsules.
• “По́иск / Poisk” (Search) and “Пирс / Pirs” (Pier) are a pair of even smaller add-ons, just four meters long, that attached to the junction ball that connects Zvezda to Zarya. Poisk sticks upward and Pirs stuck downward. Though the two are nearly identical, Pirs was mainly used as a docking port, while Poisk is designated as Mini-Research Module 2. Each has its own small arm to manage the gear stuck onto the outside — not a robot arm, but a simple telescoping crane operated by hand cranks, which reduces weight and complexity. If needed, any of the three could function as a dock, or as an airlock for going outside. Mostly Pirs has been the airlock. With all these, up to four Soyuz and Progress craft could dock — and they do often dock as many as three. These blur the line between a docking adapter and an actual module. Pirs was added in 2001, and Poisk not until 2009. Pirs was then discarded in 2021, so its port could instead be used for:
• “Нау́ка / Nauka” (Science): a large laboratory which replaced Pirs, sticking downward. It was made long ago as a backup for Zarya and is very similar to it, and its downward end likewise has a small spherical junction. Nauka’s launch was originally meant to happen as early as 2007, but defects were found; between those and the years of disuse since, extensive refurbishing was required to make it ready. It has a robotic arm made by Airbus, and adds new life support facilities, including a galley and a toilet.
• “Причал / Prichal” (Berth): a spherical junction ball stuck onto the bottom end of Nauka, which already has a junction ball. Prichal is a lot roomier than the original ball, with 19 cubic meters of air inside. It was launched a few months after Nauka on top of a Progress freight capsule. This finally brings modern international docking ports to the Russian end, so non-Russian spacecraft can park there. It was originally intended for use elsewhere, as a core part of an orbiting dockyard called OSPEK, which would host an ever-changing collection of modules and components that could, for example, be where a Mars mission sets off from, with its parts assembled in orbit. They had visions of it becoming a starting point for a whole network of space stations. The OSPEK plan was dropped in 2017.

other facilities

Where, you may ask, does the big “Canadarm” attach? The answer is, anywhere it wants to. It can swing its free end around, attach it, and detach the former base, switching end for end, and by this means it can inchworm its way all over the station, using any of several attachment points, one of which is a sort of railroad car that allows it to slide to any position along the width of the big external truss. This truss car may be its most used anchorage. Recently an attachment point was added to Zarya. There are two control stations for it, one in the Cupola and one in Destiny. The arm is 17.6 meters long and weighs about 1.6 tons, without the optional “dexterous manipulator” that it can add to the end, which for short is called Dextre, or the Canada Hand. Another add-on that can go on the end is a boom which can add fifteen more meters of reach.  The arm is capable of wrassling objects as big as a space shuttle, as long as it moves them slowly enough. (Moving anything around the outside of the station is always slow... docking and undocking can take a couple of hours, for instance, and the top speed of the truss rail car is one inch per second.)

The ESA is planning to add a smaller arm to Columbus. That would make four arms overall, with those of Kibō and Nauka.

All in all the station is 75 meters long, and the truss has a width of 109 meters. The outer parts of the truss have eight solar panel wings, each with two panels 35 meters long and about 4 wide, which together can produce up to 240 kilowatts peak power... or could when they were new. Some are now being replaced, with new better cells being rolled out on top of the old ones. The whole station masses somewhere toward 500 tons — triple the size of Mir, the next largest object ever put into orbit. There are 1032 cubic meters of pressurized volume in it, if I’ve added it up correctly — enough to hold about one ton of air, as they use full sea-level pressure — but less than 500 cubic meters in the passageways open to people, which is still the size of a house, though much longer and thinner.

It may yet get a lot bigger — several private companies have plans to attach more modules, and perhaps eventually set them free to become independent stations. The company most serious about this right now is Axiom Space, which already sells ISS tourism flights to multimillionaires. Their planned expansion has its own article, which also briefly covers some other space station proposals which as yet exist only as artists’ conceptions.

Russian plans reconsidered

The Russians had been planning to expand their end further in the near future, but then decided that the ISS was too old to keep investing in, and maybe that American astronauts have cooties. (Some distaste at sharing orbital quarters is starting to be heard among American astronauts as well, as Putin’s crimes make cordial relations increasingly impossible.) But before this decision, they did build another module which could have been ready to attach soon. It is called NEM-1 or the Science Power Module, and does not yet have a friendly name. It would provide additional large solar panels and plenty of other external equipment, along with a good amount of multipurpose habitable space. This would have stuck out from the starboard side of Nauka’s bottom junction. They originally were going to have an NEM-2 as well, presumably on the port side, but the budget got cut.

Zvezda, which is critical for life support, is showing its age, and they’re starting to feel a bit nervous about trusting cosmontauts’ lives to it. It recently sprang a leak, and while that was being fixed, some oxygen equipment broke. (They make oxygen by electrolysing recovered water vapor.) They recently had to repair the toilet as well, and its rear rocket motors may now be unusable due to equipment added next to them. Then they found that the leak they’d patched was just one of two leaks. (Also, Zvezda is an unpleasantly noisy place to live... some cosmonauts bring earplugs.) The new Nauka module has problems too: its main engine wouldn’t fire, and while it was docking a thruster fired for no reason, which made the entire station do a slow backflip. And it had leaks of its own, both when newly installed and after a couple of years in service. After that thruster mishap, they found cracks in Zarya. Because of issues like these, the whole ISS might be retired within a decade, in favor of new stations... or if those cracks spread, maybe a lot sooner.

So the Russians are now saying they will build a whole new independent station instead of expanding the old one, though to budget such a thing sounds like it might be very difficult. After a round of American diplomatic sanctions in 2021, they said they were going to pull out of the ISS at the earliest possible date, after the current term of agreement ends in 2024. After all, they say, “the station’s modules... have mostly worn out their service life.” But a couple weeks later they backed down from this attitude, which probably means they gained some concession. They now say they plan to stay until 2028... or they did before the Ukraine invasion. After the very negative response from the west, they started hinting that ISS cooperation may be ending. Eventually things smoothed down and cooperation seemingly went back to normal. Then Putin fired Dmitri Rogozin from Roscosmos, and the new guy Yuri Borisov (who we had hoped would be less of a fascist blowhard) started the noise right up again. They again announced they’d leave in 2024, but then once again backpedaled, saying they’d leave only once their new station is ready. They’re going to keep NEM-1 on the ground for now, saving it for the new station.

ROSS

If they manage to build this new habitat, its name would be Russian Orbital Service Station (Российская орбитальная служебная станция), which handily abbreviates as ROSS (РОСС), the root name of the motherland. They want to put it into a sun-synchronous polar orbit so all of Russia would be visible from it.

This would make relocating existing modules such as Nauka rather impossible, as the orbital plane change would require something like six km/s of delta-V. Nauka is arguably obsolete, as it is, like Zarya, based on a Salyut design that dates back to the nineteen seventies, so maybe they won’t be all that reluctant to write it off. It sounds like NEM-1 will become the first module, after some refitting to give it Zvezda-like core functionality. This would be followed by a second similar module which would apparently take over as core, then a node section and a gateway piece. Eventually they plan to have four large modules arranged into a plus-sign shape around a cluster of minor pieces at the center, if they still have the funds to expand that far (spoiler: they won’t).

We shall see if they hold to this plan once they get into the process of actually budgeting and constructing the new station, which even before the economy-crushing sanctions of 2022 would have been a huge stretch for what was left of Roscosmos.

American options

On the American side, there was talk for a while of adding a big centrifugal wheel up front, so people could sleep in simulated gravity for better health, but it has not been funded. NASA plans to keep the ISS going until 2031, and then transition to one or more commercially built stations. At that point they will be faced with the problem of safely disposing of the empty station before it falls on someone. They plan to have it plunge to a fiery death over the South Pacific, but controlling such a massive object to reenter correctly will not be easy. It might take twenty tons of fuel to make sure it doesn’t come down in an inhabited area.

Meanwhile, to keep the ISS running until then, they might have to replace stuff that’s currently being handled by Russian equipment... and they just might end up having to do it on short notice, and if not they may need to be ready by 2024. They might need new life support gear to recover water and oxygen, new thrusters to stabilize their orientation, and new propulsion (hopefully ion engines, like China is using) to maintain the orbit. None of these things are costly or difficult, by space standards... unless they are needed in a hurry.

The trend is toward separate stations rather than joint ones. The Russians want to put up ROSS by 2030, though it would be tough financially. India also wants to start its own station by 2030, and the European Space Agency eventually wants one as well. The Chinese station will be mature, and might be a lot bigger by then... and the Russians might just see if they can go halfsies on that, or that may be just talk. And there’s increasing speculation that Jeff Bezos’s end goal for the New Glenn is to put up numerous orbital habitats with it, as he wants to see large amounts of private industry expanding and thriving in orbit.