STEP 1 · habitation
Personal Provisions — MAG & In-Suit Drink Bag
The astronaut's water pouch and an absorbent garment, worn under everything — because a spacewalk can last most of a workday.
Why it exists: Once the suit is sealed there is no popping back inside for a drink or a bathroom break, so these go on first.
Without it: The astronaut would get dangerously thirsty long before the hours-long job is done.
For engineers: Sealed-suit EVAs of roughly seven hours demand self-contained consumables so the astronaut closes their own loop and the backpack only has to manage gas, heat, and power.
The in-suit drink bag holds 21 ounces of water, and the waste garment is, in NASA's own words, 'a modified incontinence diaper.'
Source: NASA JSC — Extravehicular Mobility Unit (EMU) Fact Sheet (FS-2019-07)STEP 2 · thermal
Liquid Cooling & Ventilation Garment
A stretchy bodysuit laced with water tubes that carries away body heat, worn like long underwear.
Why it exists: Working hard inside a sealed suit makes a lot of heat with nowhere to go, so cool water flowing through tubes keeps the astronaut from overheating.
Without it: The astronaut would cook in their own body heat and have to stop.
For engineers: It closes the astronaut's thermal loop — ventilation flow plus conductive water cooling — so the sublimator can reject metabolic and equipment heat to space.
About 300 feet of tubing is woven through the cooling garment — enough thin tube to run the length of an American football field.
Source: NASA — Spacewalk Spacesuit BasicsSTEP 3 · structure
Hard Upper Torso (HUT)
The hard shell over the chest and back — the strong core that the arms, helmet, and backpack all lock onto.
Why it exists: Everything else clips to this rigid piece, and it is the part that holds the air pressure around the astronaut's body.
Without it: There would be nothing to attach the rest of the suit to, and no sealed shell to hold air.
For engineers: The EMU runs at 4.3 psia of 100% oxygen — low enough to keep the suit flexible, which is why crews prebreathe pure O2 first to purge nitrogen and avoid decompression sickness (the bends); the rigid HUT anchors that low-pressure oxygen vessel and every interface.
The suit is modular: mix-and-match hard parts in three sizes fit crew members from a 5-foot, 110-pound person up to 6 foot 2 and 223 pounds.
Source: NASA JSC — Extravehicular Mobility Unit (EMU) Fact Sheet (FS-2019-07)STEP 4 · structure
Lower Torso Assembly (LTA)
The suit's pants and boots — the sealed lower half that connects to the hard torso at the waist.
Why it exists: It seals the legs and feet against the vacuum and gives the astronaut something to stand and anchor in.
Without it: The lower body would be open to space and the suit could not hold air.
For engineers: It closes the lower pressure volume, carries mobility bearings, and interfaces to foot restraints — the multilayer build trades single-layer simplicity for pressure integrity plus thermal and debris protection.
The flexible parts of the suit are built from as many as 16 layers of material — a soft, wearable hull.
Source: NASA — Spacewalk Spacesuit BasicsSTEP 5 · mobility
Arms & EV Gloves
The suit's arms and gloves, with bearings that let the astronaut turn and bend, and little heaters in the fingertips.
Why it exists: Hands do all the work on a spacewalk and get coldest, so the gloves are heated and jointed for grip.
Without it: The astronaut could not move their arms or grip any tool — and their fingers would freeze.
For engineers: A glove at 4.3 psi is a balloon that resists every grip, so hand fatigue — not oxygen — often paces the EVA; bearings put rotation where fabric would otherwise fold, fingertip heaters offset the extremities' rapid radiative loss, and cut-glove damage from sharp ISS handrail edges made in-EVA glove inspections routine.
The gloves have tiny fingertip heaters because the hands lose heat fastest — and some gloves are custom-molded to an individual astronaut's hands.
Source: NASA NTRS — EVA Hardware & Operations OverviewSTEP 6 · comms
Communications Carrier Assembly (Snoopy Cap)
A soft cap with built-in earphones and microphones — the astronaut's headset inside the helmet.
Why it exists: It lets the astronaut talk with Mission Control and crewmates and hear warning tones.
Without it: The astronaut would be sealed in silence, unable to hear or be heard.
For engineers: Transducer redundancy means a single failed microphone or earphone degrades, rather than severs, the only voice and caution/warning channel; the radio itself rides in the PLSS, so the head-borne assembly is just transducers and wiring — nothing to reach or adjust once the helmet is sealed.
Astronauts call it the 'Snoopy Cap' — a nod to the black-and-white aviator headgear the Peanuts beagle wears in his flying-ace daydreams.
Source: NASA JSC — Extravehicular Mobility Unit (EMU) Fact Sheet (FS-2019-07)STEP 7 · optics
Helmet & Extravehicular Visor Assembly (EVVA)
The clear bubble helmet plus a gold-tinted sun visor, shades, and lights that let the astronaut see safely.
Why it exists: Sunlight in space is blinding with no atmosphere to soften it, so a gold visor works like super-sunglasses.
Without it: The astronaut would be blinded by glare and unprotected from the Sun.
For engineers: Ventilation flow enters at the back of the helmet and washes down over the visor and face, preventing fogging and sweeping exhaled CO2 into the return path; the gold coating rejects infrared and glare, the polysulfone visor takes impacts and micrometeoroids, and helmet lights carry the work through the darkness of each 90-minute orbit.
The sun visor is coated with a thin layer of real gold, which reflects the Sun's harsh glare and infrared while still letting the astronaut see.
Source: NASA — What Is a Spacesuit? (Grades 5-8)STEP 8 · life-support
Primary Life Support System (PLSS)
The backpack — the astronaut's built-in life support: it holds the oxygen to breathe, cleans the air, cools the water, and powers the suit.
Why it exists: In space there is no air, so the astronaut has to carry their own supply of everything on their back.
Without it: No oxygen, no cooling, no power — the suit would be a lifeless shell.
For engineers: Integrating gas, thermal, power, and comms into one back-mounted module makes the suit a self-contained spacecraft; the sublimator rejecting heat to vacuum and the metabolic-oxygen loop are what set EVA duration.
The complete ISS suit — pressure garment, this backpack, and the SAFER rescue jetpack — weighs about 319 pounds (145 kg) on Earth, yet weighs nothing in orbit.
Source: NASA — What Is a Spacesuit? (Grades 5-8)STEP 9 · instrument
Display & Control Module (DCM)
The control panel on the chest — switches and a small display the astronaut uses to run the suit and see warnings.
Why it exists: The astronaut needs to control oxygen, pressure, and cooling by feel, so the buttons sit on the chest within reach of one gloved hand.
Without it: The astronaut could not adjust or monitor their own life support.
For engineers: Suit commanding stays mechanical — the oxygen actuator and the cooling-control valve are shaped to be worked by feel in a pressurized glove — while the Caution and Warning System pairs the chest display, read via the wrist mirror, with tones in the CCA so a fault registers even while the crewmember is heads-down in a task.
Every critical switch and the caution-and-warning display sit on the chest so an astronaut can run the suit one-handed, in a stiff pressurized glove, without looking away from the work.
Source: NASA NTRS — EVA Hardware & Operations OverviewSTEP 10 · life-support
Secondary Oxygen Pack (SOP)
An emergency air tank — a backup oxygen supply that switches on by itself if the main one fails.
Why it exists: If the main oxygen ever quit, the astronaut needs enough air to get safely back inside.
Without it: A single oxygen failure would leave no backup at all.
For engineers: Purge-mode flow simultaneously supplies breathing gas and flushes CO2 without the closed loop, giving a deterministic egress reserve independent of the primary system.
If the main oxygen ever fails, the Secondary Oxygen Pack kicks in on its own with at least 30 minutes of air — enough to get the astronaut back to the airlock.
Source: NASA NTRS — EVA Hardware & Operations OverviewSTEP 11 · propulsion
SAFER — Simplified Aid For EVA Rescue
A little jetpack worn under the backpack — if an astronaut ever floated away, its gas thrusters fly them back.
Why it exists: On a spacewalk you are clipped to the station, but if the tether ever failed, SAFER is the way home.
Without it: An untethered astronaut would have no way to stop drifting or get back.
For engineers: Untethered EVA has no guaranteed vehicle-based rescue, so a wearable propulsive unit with detumble and translation authority closes the self-rescue gap; it was first flight-tested untethered on STS-64 in 1994.
SAFER was first tested untethered on shuttle mission STS-64 in September 1994, when astronauts Mark Lee and Carl Meade flew free of the orbiter to prove a spacewalker could fly back home.
Source: NASA — 30 Years Ago: STS-64 Astronauts Test a Spacewalk Rescue Aid
