This Friday, astronauts Scott Kelly and Kjell Lindgren will squeeze out of a hatch on the International Space Station (ISS) to fix an ammonia cooling system last repaired in 2012. Their second spacewalk in less than two weeks will be broadcast live on NASA TV starting at 7:10AM Eastern Standard Time (12:10PM Greenwich Mean Time) on Friday, and is expected to last around six and a half hours.

Last week Kelly—who just broke the record for the longest that a NASA astronaut has lived in space—and Lindgren spent over seven hours outside of the station, where they completed a lengthy to-do list: lubing up the latching end of a robotic arm, laying the groundwork for a new docking port for Boeing and SpaceX commercial crew vehicles, and putting a thermal cover on a device used in dark matter research. All of this was done as the space station they’re attached to zipped around the earth at 5 miles (around 8 kilometers) per second.

We wanted to know more about undertaking the ultimate high-altitude hike. So we caught up with astronaut Douglas Wheelock (who prefers to be called Wheels). He’s been on six spacewalks, including an impromptu excursion in 2007 to fix a torn solar array.

Wheelock—oh, sorry, “Wheels”—took to Twitter during last week’s spacewalk to answer questions under the hashtag #askNASA. He’ll be doing the same thing this Friday. In the meantime, he spoke to National Geographic from his post as NASA’s interim Director of Operations Russia.

Here are a few out-of-this-world facts we learned from him about what it’s like to walk in space:

1. You wear vintage clothing. The spacesuits that Kelly and Lindgren will don on Friday are not the latest in celestial fashion. In fact, they’re around 35 years old, according to Wheelock, and lots of astronauts have sweated in them. These shared garments are tough to clean because they’re filled with pure (highly combustible) oxygen when they’re in use—even the lint from a towel could catch on fire if it were left behind from cleaning, which limits what the crew can do to freshen up the gear. Rest assured, everybody wears their own underwear and cooling garments, which helps. Somewhat.

“If you were to stick your head inside of there and take a smell, it probably smells like a locker room,” Wheelock says.

2. You run hot and cold. “In space we don’t have the protection of the Earth’s atmosphere,” he says, and the temperature can soar in direct sunlight near the space station’s reflective surfaces, then plummet when the sun goes out of sight. These massive fluctuations of nearly 500 degrees Fahrenheit happen a lot during a seven-hour spacewalk: The ISS’s speedy 90-minute orbit around the Earth means that it sees the sun rise and set every 45 minutes.

No amount of underwater spacewalk training in NASA’s Neutral Buoyancy Lab can prepare you for these temperature shifts, he adds.

The heat is easier to deal with than the cold. The astronauts can sometimes find shade around the space station.“You tend to overheat. If you didn’t have a cooling garment you would kind of sizzle to death in that suit,” Wheelock says.

3. It’s not really walking. After his extravehicular activity or EVA (the official NASA term for a spacewalk) on October 28th, astronaut Scott Kelly joked that it shouldn’t be called spacewalking, but spaceworking.

Wheelock agrees that “walking” is a misnomer, calling it instead a space ballet on fingertips.

“You propel yourself, you stop yourself, you hold onto things with your fingertips, and you just kind of push off and just get yourself stable with your fingertips,” he says.

So it makes sense that the sorest muscles after a spacewalk are the ones used for gripping: hands and forearms. Legs just sort of get in the way, Wheelock says.

“The best thing you could do for your spacewalking fitness is to sit with one of those hand squeezer things” he says. “You’ll have big Popeye arms and strong hands.”

4. You have to watch your language. During a spacewalk, the astronauts’ microphones are hot—meaning everyone in mission control and watching the live feed can hear every breath, every word, and every muttered curse.

The extreme temperatures and conditions in space are hard on equipment outside of the space station—so fixing them can be an exercise in frustration. Those inclined to swear a blue streak while putting together IKEA furniture are encouraged to bite their tongues and find their happy places when things get tough during a spacewalk.

“Fortunately I don’t think anyone has dropped an F-bomb or anything like that on an open mic,” Wheelock says. “But I still remember in 2010 when I was working on that pump module, I was having just a devil of a time getting this one connector off and I think sort of under my breath I said something like ‘damn.’”

He thought he was in the clear until he spoke with his parents on the phone, and his dad said, “I heard you curse.”

5. Scratching an itch requires creative thinking. When that sweaty suit gets itchy, Wheelock says that you can wriggle around enough to scratch most itches. For itchy noses, there’s a foam device astronauts press their noses against to clear their ears (the same maneuver as when you’re on an airplane and pinch your nose and blow to equalize pressure). For an itchy upper cheek, there’s the mic boom and the drink bag straw. But for an itchy forehead, it sounds like you just have to lump it.

6. And it could always be worse. “We have this saying that there’s nothing so bad in space, that you can’t make it worse,” Wheelock says. Staying in the moment instead of dwelling on past mistakes or anticipating future moves can help prevent accidents—that’s why there’s a lot of communication, he says.

The worst thing he can imagine happening is a piece of space debris or a micrometeroid hitting and tearing an astronaut’s spacesuit, making it leak oxygen. This scenario is included in astronaut training, but not every potential danger can be anticipated—like when European Space Agency astronaut Luca Parmitano’s helmet filled with water during a 2013 spacewalk, blinding him as he felt his way back to the airlock.

“Any time you open that hatch and go outside into space it is fraught with peril. Any number of things can go wrong, you just have to rely on your training, and trust your team,” Wheelock says.

7. But it’s also the best thing ever. While there are moments where Wheelock says he couldn’t wait to get back inside, they were outweighed by the moments when he never wanted it to end. Watching the green and red aurora rain down on the poles and lightning flash below him, with only the sound of his suit’s fan and the sound of his breathing in his ears, the visual splendor of the earth was overwhelming.

“The Earth is this living, breathing, ball of life in this vast empty sea, and it’s just raging with light and life and motion and color... It’s just quite amazing.”