NASA astronaut Ed White called it "the saddest moment of my life" when Mission Control ordered him back inside his Gemini 4 spacecraft after making America's first spacewalk on June 3, 1965.
It may be the ultimate test for an astronaut: Six hours spacewalking outside the safety of a spaceship while wearing what is essentially a human-shaped balloon wrapped in layers of bulky insulation. But American astronauts have been spacewalking for 45 years, leaving their mark in orbit in the shape of a gleaming $100 billion space station.
The first American spacewalk, performed by astronaut Ed White on June 3, 1965, lasted just 23 minutes when he floated outside his Gemini 4 spacecraft while secured with an umbilical cable. It came just months after the world's first spacewalk ? a harrowing 10-minute excursion ? by Soviet cosmonaut Alexei Leonov on March 18 of that year.
Since then, American astronauts have performed 239 spacewalks, each of which typically lasted six hours or more. Some of those have been on the surface of the moon, while others helped save and upgrade the iconic Hubble Space Telescope. But most have been focused on the International Space Station. [Graphic: How spacesuits have changed.]
A station in space
More than half of all spacewalks by NASA astronauts ? 146 of them ? have been aimed at building the space station, which is nearly complete after almost 12 years of orbital construction. Astronauts also routinely work alongside Russian cosmonauts and spaceflyers from other countries to assemble and maintain the station.
"It's a pretty phenomenal achievement when you think of the environment you're in and what the suit has to protect you against," said astronaut Michael Lopez-Alegria, a 10-time spacewalker and NASA's reigning spacewalking champion.
When construction began on the space station in 1998, NASA engineers and astronauts were daunted by what they called the "wall" of spacewalks required to build the outpost, piece by piece, 220 miles (354 km) above Earth. Astronauts eventually had to tackle repairs never dreamed of by early spacewalk planners, like sewing torn solar arrays back together and cleaning huge gummed up gears to keep the orbiting lab in good shape.
"We're able to do so much more now. It's amazing," Lopez-Alegria told SPACE.com. "It's like comparing flying in an airplane to a horse and cart."
Lopez-Alegria has flown four space missions, three of them on NASA shuttle flights that included five spacewalks in all. His five other spacewalks occurred while in command the International Space Station in 2006 and 2007.
To date, Lopez-Alegria has spent 67 hours and 10 minutes on spacewalks ? the most time of any American.
"It's just been a privilege and an honor," he said of his work on the space station, adding that he was in the right place at the right time to grab NASA's spacewalking title. "I think a lot of it has to do with pure luck."
The world record for most spacewalks rests firmly with Russian cosmonaut Anatoly Solovyov (16 spacewalks over 82 hours and 22 minutes).
The American spacesuit
To walk in space, American astronauts today wear a bulky white spacesuit called the Extravehicular Mobility Unit (EMU). Russia's cosmonauts use a different spacesuit, called Orlan, while China's spacewalkers use yet a third type named Feitian.
A spacewalk, in NASA parlance, is known as an extravehicular activity (EVA).
The NASA spacesuit's exterior is covered in 17 layers of insulation and tough fabric to protect against punctures and the severe temperatures swings in space, which range from 250 degrees Fahrenheit (121 Celsius) in sunlight to minus 250 F (minus 156 C) at night.
"It's like being bundled up for winter in several layers," said NASA engineer Glenn Lutz, manager of the EVA office at the agency's Johnson Space Center in Houston. ?
The spacesuit's core is its hard upper torso ? the attachment point for softer sections that form the arms and legs, as well as the life support backpack. Completing the orbital ensemble are gloves and a glass helmet, complete with video cameras, lights (for night work) and a visor to block the sun's glare.
There is even a 30-minute backup oxygen supply and an emergency jetpack powered by cold nitrogen gas in case an astronaut drifts loose from the station or space shuttle.
But none of those backups have ever been called upon in an actual U.S. spacewalk, a fact Lutz attributes to prepared astronauts and their experienced instructors and support engineers.
"I think we all understand how critical what we do is and the consequences of a failure on our parts to make sure to get the attention on the right things," Lutz said in an interview.
Astronauts practice for walking in space using virtual reality gear and a giant swimming pool at the Johnson Space Center, one big enough to fit life-size versions of a space shuttle payload bay and major segments of the International Space Station, which is long as a football field.
"They don?t bother painting the Earth on the floor of the pool or anything," Lopez-Alegria said. But since an astronaut practices several hours for every one hour of planned spacewalking time, it's ample training.
Unlike the spacesuits used on the Apollo moon landings, the current spacesuits aren't designed for actual walking. They're only built for weightless work.
"Today, if we were naming spacewalks we probably wouldn't call them spacewalks because you do not use your feet or your legs," explained Robert Pearlman, editor of collectSPACE.com, an online publication for space history and artifact enthusiasts, and a SPACE.com contributor. "Your arms become your main means of getting around."
And it's not always a comfy ride. Spacesuit shoulder joints don?t completely mesh with a human's range of arm motion, which can cause injuries during training simulations in the big pool, Lopez-Alegria said.
James McBarron, a retired NASA engineer who worked on spacesuits from the early Mercury program through the shuttle era, said some astronauts also suffered from finger and fingernail injuries while preparing for spacewalks.
"They liked the gloves to be extremely tight," McBarron told SPACE.com.
Earth through a visor
And then there are the views. The sight of Earth from space through nothing but a spacesuit helmet was tremendous to White, who later said Mission Control's order to end the spacewalk was the "saddest moment of my life."
Lopez-Alegria agreed. He still vividly remembers the colors of the Caribbean during a spacewalk on his second spaceflight (he's flown four space missions) when he helped test the emergency jetpack for NASA's current spacesuits.
"The colors of the Caribbean are spectacular anyway," Lopez-Alegria said. "But seeing that panorama and the Gulf, recognizing the geography of it, that was quite an experience."
After nearly five decades of spacewalks, NASA has racked up a long list of achievements.
Among those are: The most people on a spacewalk (3 astronauts ? they plucked an ailing free-flying satellite from space in 1992); unprecedented repairs to spacecraft like Hubble, vital space station servicing (Skylab in the 1970s and now the International Space Station) and international cooperation.
Spacewalkers literally saved Skylab, the first U.S. space station, after its launch left it crippled with a missing solar array and exterior damage, Lutz said. Astronauts helped free Skylab's remaining solar panel and set up a parasol to shield it from the sun.
"So that was a pretty big one as far as EVAs go," Lutz said. "They're all pretty amazing."
With NASA's space shuttle program retiring this year and the ability to ship bulky gear up to the space station going out with it, the agency will be left without a way to return its current spacesuits to Earth for safekeeping, Pearlman of collectSPACE.com said. Some may be discarded to burn up in Earth's atmosphere, while the rest may be kept in reserve to serve as spares or for parts.
"Unfortunately, the EMU may be the only spacesuit to date in U.S. history that will not have a flown representative in museums," Pearlman said.
NASA's future missions may send astronauts to visit an asteroid, Mars or the moon, which will require new spacesuits designed for surface work.
Lopez-Alegria, Lutz and McBarron believe that more flexibility and dexterity will be vital for those spacesuits to perform, but the final design will depend on NASA's ultimate destination.
"I think the mission use dictate the requirements that the suits will have to meet," McBarron said. ?"If you go back to the moon you'll find you need suits that can walk."