Space walk

Extra-vehicular activity (EVA) is work done by an astronaut away from the Earth, and outside of a spacecraft. The term most commonly applies to an EVA made outside a craft orbiting Earth (a spacewalk), but also applies to an EVA made on the surface of the Moon (a moonwalk). In the later lunar landing missions (Apollo 15, 16, and 17) the command module pilot (CMP) did an EVA to retrieve film canisters on the return trip; he was assisted by the lunar module pilot (LMP) who remained at the open command module hatch. These trans-Earth EVAs were the only spacewalks ever conducted in deep space.

A "Stand-up" EVA (SEVA) is where the astronaut does not fully exit a spacecraft, but is completely reliant on the spacesuit for environmental support. Its name derives from the astronaut "standing up" in the open hatch, usually to film or assist a spacewalking astronaut.

EVAs may be either tethered (the astronaut is connected to the spacecraft; oxygen and electrical power can be supplied through an umbilical cable; no propulsion is needed to return to the spacecraft), or untethered. Untethered spacewalks were only performed on three missions in 1984 using the Manned Maneuvering Unit (MMU), and on a flight test in 1994 of the Simplified Aid for EVA Rescue (SAFER). A SAFER is a safety device worn on tethered U.S. EVAs, since the capability of returning to the spacecraft is essential.

Russia, the United States and China have demonstrated the capability to conduct an EVA.

Development history

NASA planners invented the term extra-vehicular activity in the early 1960s for the Apollo program to land men on the Moon, because the astronauts would leave the spacecraft to collect lunar material samples and deploy scientific experiments. To support this, and other Apollo objectives, the Gemini program was spun off to develop the capability for astronauts to work outside a two-man Earth orbiting spacecraft. However, the Soviet Union was fiercely competitive in holding the early lead it had gained in manned spaceflight, so the Soviet Communist Party, led by Nikita Khrushchev, ordered the hasty conversion of its single-pilot Vostok capsule into a two- or three-person craft named Voskhod, in order to compete with Gemini and Apollo. The Soviets were able to launch two Voskhod capsules before the first manned Gemini was launched.

The Soviets' avionics technology was not as advanced as that of the United States, so the Voskhod cabin could not have been left depressurized by an open hatch; otherwise the air-cooled electronics would have overheated. Therefore a spacewalking cosmonaut would have to enter and exit the spacecraft through an airlock. By contrast, the Gemini capsule's avionics were designed so the cabin could be exposed to the vacuum of space when one of two large hatches was opened, so no airlock was required, and both the spacewalking astronaut and his companion command pilot were in vacuum during the EVA. Due to the different designs of the spacecraft, the American and Soviet space programs define the duration of an EVA differently. The Soviet (now Russian) definition is the time when the outer airlock hatch is open and the cosmonaut is in a vacuum. An American EVA begins when the spacewalking astronaut has at least his head outside of the spacecraft.

As they had with the first satellite and first man in space, the Soviets again stunned the world on March 18, 1965 with the first EVA (commonly referred to as a "space walk") performed by Alexey Leonov from the Voskhod 2 spacecraft, for 12 minutes outside the spacecraft. Leonov had no means to control his motion other than pulling on his 50.7-foot (15.5 m) tether. After the flight, he claimed this was easy, but his space suit ballooned from its internal pressure against the vacuum of space, stiffening so much that he could not activate the shutter on his chest-mounted camera.

At the end of his space walk, the suit stiffening caused a more serious problem: Leonov had to re-enter the capsule through the inflatable cloth airlock, 3.96 feet (1.21 m) in diameter and 8.25 feet (2.51 m) long. After his spacewalk, he improperly entered the airlock head-first and got stuck sideways. He could not get back in without reducing the pressure in his suit, risking "the bends". This added another 12 minutes to his time in vacuum, and he was overheated by 1.8 °C (3.24 °F) from the exertion. It would be almost four years before the Soviets tried another EVA. They misrepresented to the press how difficult Leonov found it to work in weightlessness, and concealed the problems encountered until after the end of the Cold War.

The first American spacewalk was performed on June 3, 1965 by Edward H. White, II from the second manned Gemini flight, Gemini 4, for 21 minutes, on a 25-foot (7.6 m) tether. White was the first to control his motion in space with a Hand-Held Maneuvering Unit, which worked well, but only carried enough propellant for 20 seconds. White found his tether useful for limiting his distance from the spacecraft, but difficult to use for moving around, contrary to Leonov's claim. However, a defect in the capsule's hatch latching mechanism caused difficulties opening and closing the hatch, which delayed the start of the EVA and put White and his crewmate at risk of not getting back to Earth alive.

No EVA's were planned on the next three Gemini flights. The next EVA was planned to be made by David Scott on Gemini 8, but that mission had to be aborted due to a critical spacecraft malfunction before the EVA could be conducted. Astronauts on the next three Gemini flights (Eugene Cernan, Michael Collins and Richard Gordon), performed several EVA's, but none was able to successfully work for long periods outside the spacecraft without tiring and overheating.

Finally, on November 13, 1966, Edwin "Buzz" Aldrin became the first to successfully work in space without tiring, on the Gemini 12 last flight. Aldrin worked outside the spacecraft for 2 hours and 6 minutes, in addition to two stand-up EVA's in the spacecraft hatch for an additional 3 hours and 24 minutes. Aldrin's interest in scuba diving inspired the use of underwater EVA training to simulate weightlessness, which has been used ever since to allow astronauts to practice techniques of avoiding wasted muscle energy.

On January 16, 1969, the Soviet Union achieved the first EVA crew transfer from one spacecraft to another when Aleksei Yeliseyev and Yevgeny Khrunov transferred from Soyuz 5 to Soyuz 4, which were docked together. This was the second Soviet EVA, and it would be almost another nine years before the Soviets performed their third.

The first EVA on the lunar surface was performed by Americans Neil Armstrong and Buzz Aldrin on July 21, 1969 (UTC), after the Apollo 11 Moon landing. This first Moon walk lasted 2 hours, 36 minutes. A total of 15 Moon walks were performed by members of six Apollo crews, including Charles "Pete" Conrad, Alan Bean, Alan Shepard, Edgar Mitchell, David Scott, James Irwin, John Young, Charles Duke, Eugene "Gene" Cernan and Dr. Harrison "Jack" Schmitt. Cernan was the last Apollo astronaut to step off the surface of the Moon.

The first EVA in deep space was made on August 5, 1971, by American Al Worden, to retrieve a film and data recording canister from the Apollo 15 Service Module on the return trip from the Moon. Worden was assisted by James Irwin, doing a standup EVA in the Command Module hatch. This was repeated by Ken Mattingly and Charles Duke on Apollo 16, and by Ronald Evans and Harrison Schmitt on Apollo 17.

The first EVA repairs of a spacecraft were made by Charles "Pete" Conrad, Joseph Kerwin and Paul J. Weitz on May 26, June 7 and June 19, 1973, on the Skylab 2 mission. They rescued the functionality of the launch-damaged Skylab space station by freeing a stuck solar panel, deploying a solar heating shield, and freeing a stuck circuit breaker relay. The Skylab 2 crew made three EVA's, and a total of ten EVA's were made by the three Skylab crews.