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Spacewalks: How they work and major milestones

Astronauts Bernard A. Harris Jr., STS-63 payload commander, (top right) and C. Michael Foale, mission specialist, exit the airlock for their spacewalk on Feb. 9, 1995.
Astronauts Bernard A. Harris Jr., STS-63 payload commander, (top right) and C. Michael Foale, mission specialist, exit the airlock for their spacewalk on Feb. 9, 1995.
(Image: © NASA/JSC)

Astronauts aboard the International Space Station (ISS) spend most of their days in the relatively safe environment of the orbiting spacecraft. But occasionally, astronauts must face the deadly rapture of space for hours, with only the thin protection of their spacesuits between them and the frigid vacuum. These jaunts are known as spacewalks.

Spacewalks were indispensable for the space station's assembly, and over two decades later, spacewalks remain a crucial component for the enormous spacecraft's continued maintenance in low Earth orbit. 

Although "spacewalk" is a slightly jazzier name, the official term for the event is "extravehicular activity," or EVA. There are many different reasons to perform an EVA, such as repairing the spacecraft, installing new pieces of equipment or deploying scientific experiments.

Related: NASA astronauts tie spacewalk record while prepping space station for upgrades

Compared to how often an astronaut performs other tasks, spacewalks are relatively rare. They're expensive, time-consuming and most of all dangerous. Astronauts usually operate robotic arms like the Canadian-built robotic arm — known as Canadarm2 — to remotely work in the harsh thermal vacuum of space. Occasionally, though, when robotics just won't do the trick, specially trained astronauts must venture outside of the spaceship to complete the task with human finesse.

How astronauts prepare for a spacewalk

Astronauts get their first space-suited glimpse of the ISS not in the vacuum of space, but 40 feet (12 meters) underwater. Years before astronauts ever perform an EVA in space, they begin training in the Neutral Buoyancy Lab at NASA's Johnson Space Center in Houston — a 6 million gallon (22,700 metric tons) pool housing a submerged replica of the space station. 

An astronaut trains for approximately 7 hours in the pool for every hour they might spend walking in space, according to NASA. Underwater, astronauts practice connecting electrical cables, removing broken equipment and everything else they'll be doing in space, all while learning how to work within the constricting suit.

But actually squeezing out of a hatch on the real ISS is not quite the same as floating in the massive training pool on Earth. In space, "there's no diver floating around to keep you safe," NASA astronaut Tracy Caldwell Dyson, who has performed three spacewalks, told NPR. Along with the ISS, spacewalkers cut through space at roughly 17,500 mph (28,000 kph). But there is no air, there is no wind and there is no sensation of speed whatsoever.

"You and the other person in the puffy white suit are the only two out there in the vacuum of space; you tend to hang on a little tighter," Dyson said.

Most spacewalks are performed in pairs. (So far, there has been only one three-person EVA.) One astronaut is the lead spacewalker, responsible for keeping track of the cadence, safety and completion of the entire jaunt. 

Related: How to become an astronaut

Once in orbit, preparation for the spacewalk starts days before the space travelers go out into the void. Astronauts prepare for their ultimate home-improvement project by checking every part of their suit and their tools, making sure they're both assembled and tethered properly.

About 24 hours before the spacewalk, astronauts undergo decompression, the same procedure divers follow when returning from the depths of the ocean to the surface of the water. Inside the space station, air is pressurized to the same degree as it is on Earth at sea level: 14.7 pounds per square inch, or 1 atmosphere. 

But inside a spacesuit it's 4.3 psi, according to NASA, which is about the same pressure experienced at 30,000 feet (9,000 meters) above Earth. Experiencing a rapid drop in pressure from 14.7 to 4.3 psi causes nitrogen bubbles to form in the bloodstream and get stuck, blocking blood flow — a condition known as "the bends" or decompression sickness. To avoid the condition, astronauts camp out the night before in a closet-sized airlock while wearing their space suit so their bodies have time to adjust to the change in pressure. 

Astronaut Bruce McCandless made the first untethered spacewalk as he flew about 300 feet from the shuttle in the first test of the Manned Maneuvering Unit on Feb. 7, 1984. The event pictured here took place a few days later on February 11. (Image credit: NASA)

How the spacesuit works

To perform spacewalks, astronauts don a bulky "Michelin Man" suit known as the Extravehicular Mobility Unit (EMU), which protects them from solar radiation, debris and extreme temperatures. While partaking in this high-altitude hike, the temperature of the surrounding environment can soar to as hot as 250 degrees Fahrenheit (120 degrees Celsius) in the sunlight and plummet to minus 250 F (minus 160 C) when the sun is out of sight. 

Spacewalking definitely isn't like a walk in the park. Everything, including the astronaut, must be tethered at all times. Astronauts mostly use their hands to get around, pulling themselves where they need to go. In case of emergency, such as an astronaut losing their tether to the spaceship, the spacesuit comes with a jet pack called SAFER (Simplified Aid for EVA Rescue), which the astronaut controls with a small joystick.

The astronaut's spacesuit is like their "own little spaceship," former NASA astronaut Mike Massimino, who performed two spacewalks, previously told Space.com. "It keeps you comfy; it allows you to do your work; it protects you." 

Related: NASA wants to test new moon spacesuits on the space station in 2023

In addition to providing layers of durable protection, the spacesuit is also flexible enough to allow astronauts to complete delicate tasks. "The repair on the Hubble that I did … required a lot of dexterity and being able to move and work and be comfortable in that suit for a long time," Massimino said. "But at the same time, it protected me from the harshness of space. I got to know that spacesuit intimately well, and it's just a miraculous piece of machinery."

Putting on the suit takes 45 minutes. The suit, which weighs 280 lbs. (130 kilograms) on Earth, is a marshmallowy ensemble of three major layers: The bladder that contains the oxygen, the restraint that provides the structure, and the thermal and micrometeoroid layer that provides protection from the environment. The suit also includes a Maximum Absorption Garment (MAG), which is essentially an adult diaper. After all, an EVA may take as long as 9 hours, and you never know when nature will call in the void of space.

The ultimate workplace distraction

Spacewalks typically last around 6.5 hours but can be extended to 7 or 8 hours, if necessary. During a spacewalk, astronauts have a laundry list of jobs they're out there to perform. Cameras mounted on their helmets document nearly every step of an EVA, and headsets help the spacewalking astronauts maintain constant contact with their spacecraft and Mission Control on the ground.

Every few minutes while floating in space, astronauts catch a glimpse of the blue marble yawning below them, experiencing a view of Earth that the majority of the planet's residents will never witness in person. Spacewalking astronauts may see the green and red auroras rain down on the poles, the deep blue ocean turning white where waves crash into sand and lightning flashing below.

Nothing compares "to that moment of opening the hatch and pulling yourself outside into the universe," Canadian astronaut Chris Hadfield told Universe Today. "I knew I couldn't keep notes up there and I would forget stuff so I sort of resolved to myself that I would verbalize and attempt to, as eloquently as I could, express what I was feeling … yet when I listen to the transcripts of what I said, most of it was just, 'Wow!' It was so pathetic! But the experience was just overwhelming!"

Major spacewalk milestones

First spacewalk

On March 18, 1965, Soviet cosmonaut Alexei Leonov made the first spacewalk in history during a 10-minute excursion on the Voskhod 2 mission, beating the Americans by almost three months.

First American spacewalk

During the Gemini IV mission in June 1965, Ed White became the first American to conduct a spacewalk. White loved every second of his 23-minute adventure: "I'm coming back in … and it's the saddest moment of my life," he said while returning to the Gemini capsule, according to a 1997 NASA monograph.

First moonwalk

NASA astronauts Neil Armstrong and Buzz Aldrin took the first toddling steps on the surface of the moon on July 20, 1969 — a high-water mark during the space race. Their moonwalk, a solid-ground EVA, lasted just under two and a half hours. 

First woman to perform a spacewalk

Svetlana Savitskaya, a Soviet cosmonaut, was the first woman to spacewalk in 1984, nearly 20 years after Leonov floated in the vacuum of space. She and crewmate Vladimir Dzhanibekov spent more than 3 hours spacewalking.

First American woman to perform a spacewalk

NASA astronaut Kathryn Sullivan became the first American woman to walk in space in October 1984. Sullivan fulfilled her dream of gazing down at planet Earth from the stillness of space: "Carl Sagan writes about the 'Pale Blue Dot' — not my experience," she told Space.com. "I did [see] the vivid blue beach ball."

First African American to perform a spacewalk

In February 1995, astronaut Bernard Harris became the first African American to perform a spacewalk. Harris dedicated his pioneering spacewalk "to all African-American achievements," the BBC reported. He also carried the flag of the Navajo Nation into space with him, to pay tribute to diverse cultures and highlight "their plight as the original Americans," the Tampa Bay Times reported

First all-female spacewalk

In 2019, NASA astronauts Christina Koch and Jessica Meir became the first pair of women to carry out a spacewalk together, replacing a failed power controller for the orbiting laboratory. 

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