A spacewalk may begin with the dramatic sight of an astronaut floating outside the International Space Station (ISS), but the work starts long before the hatch opens. Preparing to enter the vacuum of space requires careful planning, specialized equipment, and several important safety procedures.

Astronauts must prepare their bodies for lower pressure, inspect their spacesuits, and coordinate every step with the crew inside the station. Depending on the mission and equipment, these preparations can take several hours or even begin the previous day.

Why the Preparation Takes So Long

One of the main challenges of a spacewalk is the difference in air pressure. Inside the ISS, astronauts normally live in an atmosphere close to Earth's sea-level pressure, approximately 14.7 pounds per square inch (psi). The U.S. spacesuit used for spacewalks operates at a much lower pressure, around 4.3 psi.

Moving too quickly between these conditions can cause dissolved nitrogen in the body to form bubbles, potentially leading to decompression sickness. This is similar to a risk faced by divers who ascend too rapidly.

To reduce that risk, astronauts follow carefully designed oxygen prebreathing procedures. Breathing nearly pure oxygen helps remove nitrogen from their bodies before they enter the lower-pressure environment of the spacesuit.

Preparing Hours Before the Hatch Opens

Spacewalk preparations involve more than putting on a suit. Astronauts and support crew members inspect equipment, review procedures, and check communication systems, batteries, cooling supplies, and safety tethers.

On the ISS, U.S. spacewalks typically use the Quest airlock, which has separate sections for equipment preparation and depressurization.

Different prebreathing methods have been used over the years. One approach involved spending the night in the airlock at a reduced pressure of approximately 10.2 psi. Another, known as the in-suit light exercise protocol, uses oxygen prebreathing and controlled exercise to help reduce nitrogen levels without requiring an overnight stay.

The exact preparation timeline depends on the procedure, spacesuit, and mission requirements. There is no single countdown that applies to every spacewalk.

What Happens During Prebreathing

During prebreathing, astronauts breathe oxygen through a mask or their spacesuit while following a carefully timed procedure. The goal is to reduce the amount of nitrogen dissolved in their tissues before exposure to lower pressure.

Under certain protocols, astronauts perform controlled exercise as part of the process. Exercise can help nitrogen leave the body more efficiently, but its timing and intensity are carefully regulated.

Meanwhile, other crew members assist with suit preparation, communications checks, and equipment connections. Every step follows established procedures designed to reduce risks before the astronauts enter the airlock's lower-pressure environment.

Putting on a Spacesuit

A spacesuit is much more than protective clothing. The U.S. Extravehicular Mobility Unit (EMU) functions as a small personal spacecraft, providing oxygen, pressure, temperature control, and protection from the space environment.

Its major components include a rigid upper torso, arm and leg assemblies, gloves, a helmet, and a portable life-support system.

Astronauts also wear a liquid cooling and ventilation garment beneath the suit. Water circulates through small tubes in the garment to help remove excess body heat during demanding work.

Before departure, the crew checks the suit's fit, particularly the gloves, and prepares mission-specific tools, lights, cameras, and safety equipment. Proper fit matters because astronauts may spend hours gripping handrails and handling equipment in pressurized gloves.

Final Checks Inside the Airlock

Once the astronauts are suited and ready, they enter the airlock for the final preparations. The inner hatch is secured, and the airlock pressure is gradually reduced per the approved procedure.

The crew verifies suit pressure, oxygen supply, cooling performance, communications, and electrical power. Astronauts also confirm that their safety tethers and essential tools are properly secured.

Before the outer hatch opens, the spacesuits must be operating independently of the station's internal systems. Mission controllers and the supporting crew monitor the sequence closely.

Only after the required checks are complete can the astronauts open the hatch and begin their work outside.

Why Every Detail Matters

A typical ISS spacewalk lasts around six to eight hours, although the duration varies with the task and conditions. Astronauts may install equipment, replace components, route cables, or carry out maintenance on the station's exterior.

Working in a pressurized suit requires considerable effort. Movement is restricted, tools must be secured to prevent drifting, and astronauts must manage their energy throughout the operation.

The preparation process also accounts for unexpected problems. Crew members rehearse emergency procedures and establish plans for returning safely to the airlock if equipment malfunctions or conditions change.

These precautions help make complicated tasks possible in an environment where even a small equipment problem can have serious consequences.

The moment an astronaut leaves the airlock is only the visible beginning of a spacewalk. Behind it are hours of oxygen prebreathing, equipment inspections, suit checks, and coordinated teamwork. Every procedure helps protect astronauts from the challenges of vacuum, pressure changes, and demanding physical work. Spacewalk preparation may be less dramatic than floating above Earth, but it is an essential part of making every mission outside the station possible.