What Happens During an ISS Spacewalk?
An International Space Station (ISS) spacewalk, also called an extravehicular activity or EVA, is a tightly choreographed operation that lets astronauts work outside the station in microgravity.
It combines robotics, life support engineering, communications, and strict safety procedures, which makes the sequence far more complex than simply opening a hatch and floating out.
Understanding what happens during an ISS spacewalk reveals how NASA, Roscosmos, ESA, and other partners keep astronauts alive while they install hardware, repair systems, and inspect the station in the vacuum of space.
Why astronauts go outside the ISS
Spacewalks are performed when work cannot be completed by the station’s robotic arm or internal systems.
The most common EVA tasks support station maintenance, upgrades, and science infrastructure.
- Installing new hardware such as antennas, cameras, or experiment components
- Replacing failed parts on the exterior of the station
- Routing cables, fluid lines, or spare equipment
- Inspecting the hull, solar arrays, radiators, and thermal shields for damage
- Preparing docking systems for visiting spacecraft
Because every minute outside the station is costly and risky, mission planners build a detailed timeline for each task before the crew ever opens the hatch.
How a spacewalk is planned
Before an ISS EVA, flight controllers on the ground and astronauts onboard review procedure steps, tool layouts, camera angles, and abort options.
NASA mission control in Houston works with the station crew and partner control centers to verify that the schedule, power configuration, and vehicle orientation all support the outing.
Planners also study orbital lighting, communication coverage, and thermal conditions.
The ISS moves at about 28,000 kilometers per hour, so the external environment changes quickly between sunlight and darkness, affecting temperature and visibility.
A single spacewalk can be timed to maximize daylight for some tasks while avoiding thermal extremes for others.
What happens before astronauts leave the airlock?
The crew begins preparation many hours before the hatch opens.
The spacesuits used for ISS EVAs are essentially small spacecraft, known as Extravehicular Mobility Units, or EMUs.
Each suit provides oxygen, pressure regulation, cooling, communication, carbon dioxide removal, and micrometeoroid protection.
Before exit, astronauts perform suit checks, install communications gear, and verify battery power, water circulation, and oxygen supplies.
They also review the timeline, confirm the tools they will carry, and position tethers, bags, and safety equipment for rapid access.
One of the most important pre-breathing steps is reducing nitrogen in the body to minimize the risk of decompression sickness, sometimes called “the bends.” Astronauts may spend time breathing oxygen and managing pressure changes before entering the vacuum environment.
How does the airlock work during an ISS spacewalk?
The ISS airlock is the transition point between pressurized station interior and open space.
Astronauts enter the airlock, seal the hatch, and then depressurize the chamber in a controlled sequence.
This gradual pressure drop protects the crew and the station hardware.
Once pressure reaches vacuum, the crew checks suit systems again, verifies communications with mission control, and opens the exterior hatch.
The astronauts then move outside using handholds, foot restraints, and tethers to stay connected to the station.
What happens during the EVA itself?
During the spacewalk, astronauts follow a step-by-step job plan that can last several hours.
Every movement is deliberate because floating in microgravity makes even simple actions slow and physically demanding.
Worksite setup
The first phase often involves moving to the correct location on the station truss or module.
Astronauts use the robotic arm, if needed, to position themselves near the worksite and reduce travel time.
They secure themselves with tethers and lock their boots or hands into fixed restraints when precision is required.
Tool handling and task execution
Tools are designed for gloved use and are usually tethered to the astronaut’s suit or a worksite bag.
Common EVA tools include torque-limited wrenches, camera systems, specialty connectors, and multi-use handles.
Because reaction forces can push an astronaut away from the station, even tightening a bolt requires controlled technique.
Tasks may include removing covers, connecting cables, installing brackets, or swapping out external components.
Mission control monitors each step and updates the timeline based on how quickly the astronauts complete the work.
Constant communication
Throughout the EVA, astronauts maintain voice communication with the crew inside and ground teams below.
These communications are essential for safety, troubleshooting, and timing.
If a tool jams or a suit reading changes unexpectedly, mission control can guide the crew through contingency procedures.
What keeps astronauts safe outside the station?
Safety during a spacewalk depends on multiple redundant systems.
The spacesuit is the main life-support layer, but procedures and hardware backups are equally important.
- Tethers keep astronauts physically connected to the ISS
- Suit life-support systems provide oxygen and remove carbon dioxide
- Cooling garments help prevent overheating under intense physical effort
- Helmet visors and thermal layers protect against solar radiation and temperature swings
- Mission control monitors suit telemetry for pressure, battery, and oxygen status
In an emergency, astronauts can retreat to the airlock, secure the hatch, and repressurize the chamber.
EVA timelines always include contingency steps for unexpected suit issues, lost tools, or task delays.
Why moving in space is harder than it looks
Although astronauts are weightless, they still have mass, so every push and pull creates a reaction.
That means a small shove can set an astronaut drifting, and a hard turn can become difficult to stop.
To manage this, spacewalkers move slowly, use stable body positions, and rely on handrails and restraints.
Thermal conditions also affect the work.
One side of the ISS may be in direct sunlight while another is in shadow, creating large temperature differences.
The suit’s thermal control systems help, but astronauts still plan their route and work pace around heating and cooling limits.
How long does an ISS spacewalk last?
A typical ISS spacewalk lasts about six to seven hours from hatch opening to closure, though the total time outside can vary.
Some EVAs are shorter if the task set is small, while others run longer when hardware is difficult to access or an unexpected issue appears.
The astronauts also spend substantial time preparing before the hatch opens and recovering afterward.
So the full operational day of an EVA is much longer than the time spent floating outside.
What happens after astronauts come back inside?
After the EVA work is complete, astronauts return to the airlock, close the exterior hatch, and slowly repressurize the chamber.
Once the pressure is restored, they remove helmets and gloves, secure tools, and begin post-spacewalk checks.
Ground teams and the crew then inspect suit data, review any problems, and confirm that all external tasks were completed correctly.
The astronauts may also perform physical checks for fatigue, dehydration, or minor suit-related issues.
The station team uses the post-EVA review to update future procedures and improve safety for the next mission.
How astronauts train for ISS spacewalks
Training for an ISS EVA takes place in facilities like NASA’s Neutral Buoyancy Laboratory, where astronauts practice underwater in full-scale station mockups.
The water environment simulates the resistance and coordination challenges of working in microgravity.
Astronauts also train in virtual reality systems, study station layouts, rehearse emergency procedures, and practice with the exact tools they will use in orbit.
This repetition is crucial because once a spacewalk begins, there is no easy way to improvise a new plan.
Why ISS spacewalks matter
ISS spacewalks keep the station operational, support international science, and prepare the way for future missions to the Moon and Mars.
They are also a proving ground for life-support systems, robotics integration, human endurance, and mission planning in the harshest environment humans routinely work in.
Every EVA is a combination of engineering discipline and human adaptability, which is why the answer to what happens during an ISS spacewalk is much more than “astronauts go outside.” It is a coordinated sequence of preparation, precision work, constant monitoring, and careful return to the safety of the station.