STEP 1 · power
Zarya (FGB, Functional Cargo Block)
Zarya was the very first piece of the space station, and it came with its own batteries and little solar wings.
Why it exists: You need power and something to plug into before you can add anything else, so Zarya went up first.
Without it: With no first block floating in orbit, there is nothing at all for the next pieces to hook onto.
For engineers: It served as the passive-to-active bridge that let Unity berth and Zvezda dock while ground teams staged the later elements.
Zarya means 'sunrise' in Russian — a fitting name for the first light of a station that would orbit into a sunrise every 90 minutes.
Source: NASA — Zarya ModuleSTEP 2 · structure
Unity (Node 1)
Unity is a six-sided hub with a door on every face, so other rooms can plug straight into it.
Why it exists: The station is made of many rooms, and they all need a central junction that joins them together.
Without it: Take away the connecting hub and the American rooms have nowhere to attach.
For engineers: It routes power, data and fluids between segments and provides the berthing ports Destiny, Quest and Node 3 all depend on.
Unity carries over six miles of electrical wiring — the nervous system that lets both halves of the station talk to each other.
Source: NASA — Unity ModuleSTEP 3 · life-support
Zvezda (Service Module)
Zvezda is where the first crews lived, ate and slept, and its engines give the whole station a push to stay up high.
Why it exists: The thin air up there slowly drags the station down, so it needs engines to nudge itself back up now and then.
Without it: With no engines to reboost it, the station slowly sinks lower and lower until it falls back to Earth.
For engineers: Its thrust budget offsets aerodynamic drag near 400 km altitude, holding the station up between visiting-vehicle reboosts.
Zvezda's core structure was actually built in the 1980s for the Soviet Mir-2 station before it was repurposed to become the heart of the ISS.
Source: NASA — Zvezda Service ModuleSTEP 4 · attitude
Z1 Truss & Control Moment Gyros
The Z1 truss carries four heavy spinning wheels that quietly turn the whole station without using any fuel.
Why it exists: The station has to keep facing the right way so its solar wings catch the Sun, and spinning wheels do that for free.
Without it: Without the spin-wheels the station can only steer by burning precious fuel, and its solar wings stop facing the Sun.
For engineers: The CMG cluster absorbs gravity-gradient and aerodynamic torques so the station holds attitude without burning thruster propellant.
Each control moment gyro spins a heavy wheel at about 6,600 rpm; together they steer the whole station using momentum alone, no fuel burned.
Source: NASA — Integrated Truss StructureSTEP 5 · power
P6 Truss & Solar Arrays
P6 unfolded the station's first giant solar wings, each about as long as three school buses in a row.
Why it exists: The station runs on electricity, and huge solar wings are the only way to make enough of it out in space.
Without it: Without the big wings there is nowhere near enough power, and the lights and computers fade to black.
For engineers: It supplied primary power from atop Z1 during early assembly and moved to the far port truss in 2007; DC-to-DC converter units step the primary bus down to a regulated 124 V for station loads.
P6 first flew perched on top of the Z1 truss for early power, then was moved all the way out to the far port end of the station in 2007.
Source: NASA — Solar Arrays on the International Space StationSTEP 6 · habitation
Destiny (U.S. Laboratory)
Destiny is the American science room, packed with racks of experiments and a big crystal-clear window.
Why it exists: The whole point of the station is to do science you cannot do on the ground, and this is where much of it happens.
Without it: Without the lab the station is mostly just a place to live, with far fewer experiments to run.
For engineers: It anchors U.S. research and station command, distributing power and thermal control to its payload racks.
Destiny holds the highest optical-quality window ever flown on a crewed spacecraft, used by the WORF rack for Earth observation.
Source: NASA — Destiny Laboratory ModuleSTEP 7 · robotics
Canadarm2 (SSRMS)
Canadarm2 is a giant robot arm that grabs new modules and visiting ships and connects them to the station.
Why it exists: Many of the pieces are far too big for astronauts to move by hand, so a huge arm does the heavy lifting.
Without it: Without the arm there is no way to catch and berth most of the pieces that come after it.
For engineers: It performs berthing of pressurized elements and free-flyer capture, and services external payloads along the truss.
Canadarm2 has no fixed base — it walks hand-over-hand between grapple fixtures, using either end as its anchor point.
Source: Canadian Space Agency — About Canadarm2STEP 8 · structure
Quest Joint Airlock
Quest is the station's front door for spacewalks, where astronauts suit up and step out into space.
Why it exists: Astronauts have to go outside to build and fix the station, and they need a special airtight doorway to do it.
Without it: Without this airlock the U.S. crew cannot easily go outside to work in their spacesuits.
For engineers: It supports both EMU and Orlan suits, and the 10.2 psi 'campout' protocol — crew sleeping in the equipment lock at reduced pressure — shortens the pure-oxygen pre-breathe needed before dropping to the EMU's 4.3 psi.
Quest can support both American and Russian spacesuits, which is exactly why it is called a 'joint' airlock.
Source: NASA — Quest Joint Airlock (STS-104)STEP 9 · thermal
S0/S1/P1 Truss & Radiators
These middle truss pieces carry big white radiators that dump the station's heat out into space.
Why it exists: All the computers and the crew make heat, and in space the only way to lose heat is to radiate it away.
Without it: Without the radiators the station bakes in its own heat and gets dangerously hot inside.
For engineers: Interface heat exchangers couple the internal water loops to the ammonia loops, which can reject on the order of 70 kW to space — sized to hold avionics and cabin loops within limits through full sun and eclipse.
The station's cooling loops circulate anhydrous ammonia, which stays liquid across the extreme temperature swings between orbital day and night.
Source: NASA — Integrated Truss StructureSTEP 10 · structure
Harmony (Node 2)
Harmony is a second hub that adds more doors for the international science rooms to plug into.
Why it exists: To add labs from Europe and Japan, the station needed a fresh junction with extra ports.
Without it: Without this hub the European and Japanese labs have nowhere to attach.
For engineers: It routes truss power, thermal and data to the forward modules and hosts the PMA/IDA ports for crew vehicles.
Harmony's name was chosen through a NASA competition entered by more than 2,000 U.S. school classrooms.
Source: NASA — Harmony ModuleSTEP 11 · habitation
Columbus & Kibo
Columbus is Europe's science room and Kibo is Japan's — and Kibo is the biggest room on the whole station.
Why it exists: The station belongs to many countries, and each partner brings its own lab to run its own experiments.
Without it: Without them, Europe and Japan lose their own places to do science in space.
For engineers: They add partner payload racks and, via Kibo's airlock and Exposed Facility, accommodation for external experiments.
Kibo, meaning 'hope' in Japanese, is the single largest module on the station and even has its own small robotic arm and airlock.
Source: ESA — Columbus laboratorySTEP 12 · power
S6 Truss & Final Arrays
S6 unfolded the last pair of giant solar wings, completing the station's full set of eight.
Why it exists: The growing station kept adding labs and needed even more power, so the final wings finished the job.
Without it: Without the last wings the station never reaches full power for all of its labs.
For engineers: Averaged across sun and eclipse the completed arrays deliver roughly 84–120 kW of usable power, covering the added loads of Columbus, Kibo and six-crew operations with battery margin.
With S6 in place the station reached its full ~2,500-square-metre spread of solar arrays — a solar-array wingspan longer than a Boeing 777's.
Source: NASA — Integrated Truss StructureSTEP 13 · habitation
Cupola
The Cupola is a bay window with seven panes where astronauts watch Earth and drive the robot arm.
Why it exists: You can't grab a visiting ship or watch a spacewalk if you can't see it, so the station needs a great window.
Without it: Without the window, astronauts are working blind when they steer the arm or greet a docking ship.
For engineers: Each fused-silica window carries an external shutter, kept closed against micrometeoroid and debris strikes except during operations; direct sightlines back up the camera views used for capture and berthing.
The Cupola's central circular window is 80 centimetres across — the largest window ever flown in space.
Source: NASA — CupolaSTEP 14 · structure
Docking Adapters (PMA/IDA)
These docking adapters are the parking spots where crew ships like Dragon and Starliner connect.
Why it exists: A new spaceship needs a matching port to dock to, so the station added adapters that fit them.
Without it: Without the adapters, the newest crew ships have nowhere to park.
For engineers: IDSS soft capture plus the IDAs' docking targets and laser retroreflectors let Crew Dragon and Starliner dock autonomously, instead of being grappled and berthed by the arm like cargo vehicles.
IDA-1 was lost in a 2015 launch failure, so IDA-2 became the first international docking adapter to actually reach the station.
Source: NASA — Meet the International Docking Adapter
