NASA - National Aeronautics and Space Administration

"It's the saddest moment of my life."

That's how America's first spacewalker, astronaut Ed White, described the moment when he was ordered to climb back inside his Gemini spacecraft, after spending more than half an hour floating freely in space. Those who have experienced a spacewalk, more properly known as extravehicular activity, or EVA, would agree. Spacewalking is an amazing experience, and there's nothing else remotely like it. The view is breathtaking. Darkness and light fly by. There's a unique sense of freedom. Spacewalkers are even treated to something truly unusual: the smell of space.

Image to left: Astronaut Ed White is shown on his first spacewalk. Credit: NASA

"The views from space are pretty spectacular," said astronaut Rex Walheim. He performed two spacewalks on his first flight, the STS-110 mission of the Space Shuttle Atlantis in April 2002. "I could see from Wyoming all the way to Southern California," he said. Walheim said that he was amazed at how blue the ocean looked from space. Owen Garriott, who performed three EVAs during the second manned Skylab mission, compared the view to looking "down this very long elevator shaft" to the surface of the Earth. "It's quite an interesting view," he said, adding that one of the most memorable vistas came as the station was crossing the Pacific Ocean toward South America. He could see from the Pacific, across the Andes Mountains, to the Atlantic Ocean.

While they said their training prepared them well for walking in space, some astronauts said one of the things that surprised them most about the actual experience was the rapid alternation of day and night as they orbit the Earth every hour and a half. "The lights keep going on and off every 45 minutes," Walheim said. John Herrington, who performed three EVAs totaling almost 20 hours on the STS-113 mission of the Space Shuttle Endeavour, said that without the filter of the Earth's atmosphere, the days and nights were "brilliant white light and incredible dark."

Image to right: This astronaut used headlamps during a spacewalk. Credit: NASA

Moving around on a spacewalk, he said, was a lot like being underwater. "It's a lot like the pool, actually," he said, adding that the neutral buoyancy tank at NASA's Johnson Space Center prepared him well for the experience. According to another astronaut, "The weightlessness of space is like being in water, without any water" -- all of the freedom of floating, without the effort of pushing yourself through water.

Another EVA experience not simulated in training, Herrington said, is the smell of space. He explained that when astronauts return from spacewalks and remove their helmets, they can smell an odor lingering on their suits from when they were outside -- a smell somewhat like burnt metal. "You'll never forget the smell," he said. "It's a neat smell."

Of course, there are a few challenges involved in performing a spacewalk. Since the space suit is pressurized, it requires some effort to move the fingers of the glove. It's not difficult at first, but after several hours, an astronaut's hands can get tired and sore. The sensation has been compared to squeezing a tennis ball for hours at a time. In addition, some astronauts have reported that it can initially be disorienting looking through a space suit helmet. Since the faceplate of the helmet is curved, it can distort the astronaut's vision.

Cosmonaut Alexei Leonov was the first person to conduct a spacewalk. On March 18, 1965, on the Soviet Union's Voshkhod 2 mission, Leonov disembarked from the two-man craft and spent about 20 minutes floating in space tethered to the Voshkhod, while fellow cosmonaut Pavel Belyayev piloted the craft. At the end of his spacewalk, Leonov encountered a problem when he attempted to return to his spacecraft. In the vacuum of space, his pressurized space suit had enlarged slightly and had become too rigid to allow him to pass through the airlock. After letting some of the air out of his space suit, Leonov was barely able to climb back into the craft.

Two and a half months later, astronaut Ed White made the first American EVA on NASA's Gemini 4 mission, commanded by James McDivitt. During the June 3, 1965, spacewalk, White spent a total of 36 minutes floating outside the spacecraft, outlasting Leonov's spacewalk. White's space suit was smaller than later EVA suits because it did not carry its own oxygen supply. Instead, White was connected by a hose to an oxygen supply on the Gemini capsule. Bundled with the oxygen hose were electrical and communication wires and a safety tether. While outside the spacecraft, White was able to maneuver himself for a while using a handheld air-pressure gun, but relatively quickly expended its supply of gas.

Image to right: An astronaut worked with the Hubble Space Telescope during a spacewalk. Credit: NASA

While walking in space may sound like a lot of fun, for the astronauts, it's all work. In the early days of spaceflight, EVAs were used to recover or change out items on the exterior of spacecraft. During the Skylab space station program 30 years ago, astronauts conducted spacewalks to repair the orbital laboratory, which had been damaged during its launch atop a Saturn V rocket. In the Space Shuttle program, astronauts performed EVAs to capture, repair and deploy satellites. Perhaps the best-known spacewalks were the servicing missions to the Hubble Space Telescope, which has been improved several times during its lifetime by astronauts. Space Shuttle flights also featured EVAs to test new techniques and equipment for spacewalking. Today, spacewalks are used to construct the International Space Station (ISS) and to conduct repairs outside the ISS. In fact, the assembly of the ISS will require two and a half times as many spacewalks as were conducted in all of spaceflight history before construction began, for a total of about 160.

Image to left: Backdropped against the blue and white Earth 130 nautical miles below, astronaut Mark C. Lee tests the new Simplified Aid for EVA Rescue (SAFER) system. Credit: NASA

During spacewalks, astronauts must wear special pressurized space suits. NASA's space suits are called Extravehicular Mobility Units, or EMUs. These suits are essentially self-contained spacecraft, with their own atmosphere and life support systems. With the addition of a Simplified Aid For EVA Rescue backpack, the suits even have their own propulsion system. Because the air pressure in the EMU is lower than that of a spacecraft, astronauts must breathe pure oxygen for hours before a spacewalk. This prevents them from getting decompression sickness, also known as the "bends," which occurs when nitrogen bubbles form in the blood. The suits used on the ISS can be resized in orbit to fit different crew members and are designed to be used 25 times before they must be brought back down to Earth for refurbishment. In addition to the American EMUs, astronauts on the ISS also use Russian Orlan EVA space suits.

Astronauts on spacewalks use tools that were specially designed for use with bulky space suit gloves. When working on the ISS, astronauts have a couple of ways to get around. One option is to move themselves around the exterior of the ISS using a series of handgrips. The astronauts remain safely tethered, so there's no risk that they could float off if they lose their grip. The other option is to ride on the robot arm. The crew members fasten their feet to the end of it and allow it to carry them to where they need to be.

Today's International Space Station is very much a work in progress. And, when the Space Shuttle begins flying again, the ISS will once more become a construction zone. But, construction work has never been as much fun as it is for a spacewalking astronaut.