CNN  — 

When astronauts venture outside of the International Space Station to go on spacewalks, the most important thing they have to do is focus.

This may sound simple, but imagine trying to focus on a memorized set of tasks while stepping out of an airlock and wearing a 300-pound spacesuit – with the glow of planet Earth and the sun and the dark void of the universe all around you. A tether connects you to the space station, and the absence of gravity keeps you from falling.

“There’s a lot of things that you really need to do, one of which is just keep your focus, even though it’s amazing out there,” said NASA astronaut Mike Fincke. “It’s really truly breathtaking. The only thing between you and the rest of the universe, seeing the whole cosmos of creation, is the glass faceplate of your visor on your helmet, and it’s just awe-inspiring.”

Astronaut Mike Fincke conducted a spacewalk on August 3, 2004, while wearing the Russian Orlan spacesuit. You can see Earth behind him.

Depending on the orientation of the space station, which completes 16 orbits of the Earth each day while moving at 17,500 miles per hour, our planet can appear above or below the astronauts.

Fincke is a veteran of spaceflight. He’s spent 382 days in space, and he’s gone on nine spacewalks in Russian and American spacesuits. Fincke is training in Texas for his fourth spaceflight and will launch to the space station later this year on the first crewed experimental test flight of Boeing’s Starliner.

More than 550 people have been to space and about half of them have been on a spacewalk, Fincke said. Spacewalks are often referred to as EVAs, or extravehicular activities.

The first spacewalk by an American astronaut was conducted by NASA astronaut Ed White on June 3, 1965. He left the Gemini 4 capsule at 3:45 p.m. ET and remained outside of it for 23 minutes. (Soviet cosmonaut Aleksei A. Leonov completed the world’s first spacewalk on March 18 of that year.)

Astronaut Ed White during the first American spacewalk.

Gemini 4 circled the Earth 66 times in four days. During the spacewalk, White began over the Pacific Ocean near Hawaii and went back inside the capsule as they flew over the Gulf of Mexico.

He exited the spacecraft using a hand-held oxygen-jet gun to push himself out, attached to a 25-foot safety tether. NASA astronaut James McDivitt, on the mission with White, took photos of White in space from inside the capsule.

White later said the spacewalk was the most comfortable part of the mission, and said the order to end it was the “saddest moment” of his life, according to NASA.

Spacewalks: Part of the job

Spacewalks are part of life on the space station. The orbiting laboratory, which has served as a home away from home in low-Earth orbit for astronauts over the past 20 years, requires routine maintenance, upgrades and sometimes, emergency repairs.

These walks usually involve two astronauts working outside of the station for about six-and-a-half hours.

But countless hours of training and preparation precede any spacewalk experience to keep astronauts safe.

“A spacewalk is probably the most dangerous thing that we do,” Fincke said. “I think it’s more dangerous than launching and landing, even though those are really tricky things. If we become separated from the International Space Station, it’s super dangerous. Now we have ways to prevent that because it is dangerous, but going outside is an extremely dangerous thing to do.”

Spacewalks are also incredibly hard and physically demanding, despite the fact that gravity isn’t weighing them down.

“You have to remember a lot of things, you have to memorize a lot of things, you have to think in real time, and, by the way, you’re moving that 300-pound piece of equipment around your body and every movement that you make is physically demanding,” he said. “And to be able to do that in a calm, cool and collected way while breathing pure oxygen that’s only 1/3 or 1/4 of the atmospheric pressure of planet Earth with everybody’s eyes on you and trying to make sure you do the job. That is a really tough day.”

All of the hard work pays off. So far, every spacewalk has been conducted safely.

Preparing to walk in space

Training on the ground before spaceflight helps astronauts focus on knowing their spacesuits, tools and understanding the tasks they’ll be asked to perform.

“Each spacesuit is its own little spaceship,” Fincke said. “It has its own electrical power and thermal control system, oxygen and everything you need to survive for six to eight hours. We have to know our equipment just like anybody who goes on an expedition mountain climbing or underwater. It’s what keeps you alive, so we need to know our equipment very well.”

Understanding how to maneuver in the suit can also help astronauts prepare for what it’s like to handle tools in space while wearing big, thick gloves in a pressurized suit.

Additionally, there’s a choreography to planning the movements of two people working outside of the space station, at times together and other times on separate tasks.

But how can astronauts possibly prepare for a spacewalk on Earth?

Pool time

They do it by plunging into the deep end of the pool – otherwise known as NASA’s Neutral Buoyancy Laboratory in Houston. It’s similar for crew training in Russia as well at the Hydrolab.

“The reason why we train underwater is because we actually wear real spacesuits, we do real tasks and we feel like we are weightless, when actually we’re just neutrally buoyant so we’re floating,” Fincke said.

Neutral buoyancy means they aren’t at the top or the bottom of the pool, so for astronauts, it feels like they’re outside in space. Fincke said training in this kind of lab prepared him so well for his first spacewalk that it felt almost exactly the same. The training is that intensive and comprehensive, he said.

Naturally, some things can’t be simulated, such as the light and temperature changes astronauts experience during their spacewalks. The station’s orbit around Earth can expose the astronauts to blinding, hot light from the sun or plunge them into cold darkness, moving from one extreme to the other. While the spacesuits protect them from the extreme temperatures, the astronauts can still feel a shift.

Spacewalk veterans can act as instructor astronauts to those in training. One thing Fincke often shares with astronauts preparing for spacewalks is understanding their spacesuits inside and out.

Exiting the space station’s airlock and going outside isn’t a quick process; astronauts have to breathe pure oxygen for a while beforehand to avoid decompression sickness, or “the bends.”

During Fincke’s first spacewalk, he breathed pure oxygen at a high rate for 30 minutes, closed his valve and went outside. But the valve didn’t actually close, causing him to rapidly lose oxygen and the spacewalk was cut short.

“Had I been listening or had I understood my suit more, I could have actually heard the oxygen flow in at a slightly higher rate,” Fincke said. “Sometimes it’s those little things that you hear that can make make a difference.”

Fincke is pictured during one of his spacewalks in 2011, which lasted for seven hours and 24 minutes.

They figured out a solution and completed the walk a few days later. This whole scenario is actually part of an episode of the children’s show “Arthur.” Fincke appeared as an animated version of himself in an episode called “Buster Spaces Out” as an example to kids about working together to come up with a solution.

Ground control

The ideal amount of time to plan a spacewalk is six months, according to Sarah Korona, EVA flight controller at NASA’s Johnson Space Center in Houston. Of course, there are extremes, like a spacewalk she worked on planning for two years while they waited for a piece of hardware to launch, or two days if there’s an emergency outside of the space station.

If you’ve ever watched a spacewalk, she’s one of the people you’ll see sitting in what’s called the “front room” on the ground. That’s because there are many people working behind the scenes in backrooms to monitor every aspect of the space station, astronauts and spacewalk to provide support. Flight controlling is truly a team effort, she said.

Korona and her EVA team build relationships with the astronauts and even get to know their mannerisms because they work together on the ground for years in training. When the flight controllers learn that something outside the space station needs to be fixed, removed, replaced or installed, they plan what hardware is needed, determine the tools required and begin choreographing a plan to accomplish the tasks.

The plan is run in the Neutral Buoyancy Lab and crew members provide feedback to ensure that they’ve created the most efficient spacewalk.

There’s a flurry of activity on the ground and the space station the week leading up to a spacewalk. On the station, the astronauts prepare their suits, life support systems and tools. They have video conferences with the ground teams to go over every detail of the task plan.

The American spacesuits essentially come in parts, so the astronauts can customize them for the best fit. On spacewalk day, they get up early and their fellow crew members on the space station help them suit up. These crew members will also monitor them from inside the space station during the walk.

The ground team works in multiple shifts to ensure that every aspect leading up to and after the spacewalk is covered. A normal shift for a flight controller is about nine hours and it’s very taxing, Korona said. Flight controllers follow a plan of procedures about 30-pages thick for the astronauts step by step, but there are also contingency plans in case anything goes wrong.

Fincke is seen during one of his spacewalks in 2004.

Video views from the helmet cameras and those outside of the space station help the flight controllers monitor what’s happening.

About every 90 minutes, the astronauts check their gloves and helmet absorption pad, or HAP, to make sure there is no water inside their helmets or tears in the gloves. Teams also monitor the astronauts’ consumables – oxygen, water for cooling, battery power and carbon dioxide removal.

This can determine the length of the spacewalk if these start to run out.

Spacewalks of the future

Spacewalks are crucial to maintain the space station, but the knowledge gained during these outings can inform the way astronauts approach repairs to their own spacecraft as we push the boundaries of exploration.

With NASA’s Artemis program, which aims to land the first woman and next man near the lunar south pole by 2024, humans will be staying on the moon for longer than the short visits Apollo astronauts experienced. As we explore the moon and eventually Mars, astronauts will need to be able to repair and maintain their suits, spacecraft and habitats.

With Fincke’s upcoming mission, he hopes to add another spacewalk to his already impressive list. He said each of his own spacewalks have been memorable, but some moments tend to stand out more than others.

During one of his spacewalks, the two crew members finished solving a problem outside of the station and asked teams on the ground what they should do with their remaining time outside. They were asked to take photographs outside of the station because it’s impacted by micrometeorites and other things in space – something the ground team wanted to track.

Fincke took pictures of the space station until it grew dark as they moved into an orbital nighttime. Unable to take photos, Fincke securely clamped himself to the outside of the station and watched as they flew over the dark side of Earth. The universe was his scenery.

Astronauts have such a packed schedule of tasks when they exit the space station that there are really only seconds or moments when they can stop and appreciate the view.

Fincke said he’ll never forget those 23 minutes for the rest of his life.