Extravehicular activity (EVA) is any activity done by anastronaut inouter space outside aspacecraft. In the absence of a breathableEarthlike atmosphere, the astronaut is completely reliant on aspace suit for environmental support. EVA includesspacewalks andlunar orplanetary surface exploration (commonly known from 1969 to 1972 asmoonwalks). In a stand-up EVA (SEVA), an astronaut stands through an open hatch but does not fully leave the spacecraft.[1] EVAs have been conducted by theSoviet Union/Russia, the United States, Canada, theEuropean Space Agency and China.
On March 18, 1965,Alexei Leonov became the first human to perform a spacewalk, exiting theVoskhod 2 capsule for 12 minutes and 9 seconds. On July 20, 1969,Neil Armstrong became the first human to perform a moonwalk, outside his lunar lander onApollo 11 for 2 hours and 31 minutes. In 1984,Svetlana Savitskaya became the first woman to perform a spacewalk, conducting EVA outside theSalyut 7 space station for 3 hours and 35 minutes. On the last three Moon missions, astronauts also performed deep-space EVAs on the return to Earth, to retrieve film canisters from the outside of the spacecraft. American AstronautsPete Conrad,Joseph Kerwin, andPaul Weitz also used EVA in 1973 to repair launch damage toSkylab, the United States' first space station.
EVAs may be either tethered (the astronaut is connected to the spacecraft; oxygen and electrical power can be supplied through anumbilical cable; no propulsion is needed to return to the spacecraft), or untethered. Untethered spacewalks were only performed on three missions in 1984 using theManned Maneuvering Unit (MMU), and on a flight test in 1994 of theSimplified Aid For EVA Rescue (SAFER), a safety device worn on tethered U.S. EVAs.
NASA planners invented the termextravehicular activity (abbreviated with the acronym EVA) in the early 1960s for theApollo program to land humans on the Moon, because the astronauts would leave the spacecraft to collectlunar material samples and deploy scientific experiments. To support this, and other Apollo objectives, theGemini program was spun off to develop the capability for astronauts to work outside a two-person Earth orbiting spacecraft. However, theSoviet Union was fiercely competitive in holding the early lead it had gained in crewed spaceflight, so theSoviet Communist Party, led byNikita Khrushchev, ordered the conversion of its single-pilotVostok capsule into a two- or three-person craft namedVoskhod, in order to compete withGemini andApollo.[2] The Soviets were able to launch two Voskhod capsules before U.S. was able to launch its first crewed Gemini.
The Voskhod'savionics required cooling by cabin air to prevent any kind of overheating, therefore anairlock was required for the spacewalkingcosmonaut to exit and re-enter the cabin while it remained pressurized. Unusually, and by contrast, the Gemini avionics did not require air cooling, allowing the spacewalking astronaut to exit and re-enter the depressurized cabin through an open hatch. Because of this, theAmerican andSoviet space programs developed different definitions for the duration of an EVA. The Soviet (nowRussian) definition begins when the outer airlock hatch is open and the cosmonaut is invacuum. An American EVA began when the astronaut had at least their head outside the spacecraft.[3] The U.S. has changed its EVA definition since.[4][5]
The first EVA was performed on March 18, 1965, by Soviet cosmonautAlexei Leonov, who spent 12 minutes and 9 seconds outside theVoskhod 2 spacecraft. Carrying a white metal backpack containing 45 minutes' worth of breathing and pressurization oxygen, Leonov had no means to control his motion other than pulling on his 15.35 m (50.4 ft) tether. After the flight, he claimed this was easy, but hisspace 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.[6]
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, 1.2 m (3 ft 11 in) in diameter and 2.5 m (8 ft 2 in) long. 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 (3.2 °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 inweightlessness and concealed the problems encountered until after the end of theCold War.[6][7]
The first American spacewalk was performed on June 3, 1965, byEd White from the second crewedGemini flight,Gemini IV, for 21 minutes. White was tethered to the spacecraft, and his oxygen was supplied through a 25-foot (7.6 m)umbilical, which also carried communications and biomedical instrumentation. He was the first to control his motion in space with aHand-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.[6] 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.[8]
No EVAs were planned on the next three Gemini flights. The next EVA was planned to be made byDavid Scott onGemini VIII, 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, andRichard Gordon), performed several EVAs, but none was able to successfully work for long periods outside the spacecraft without tiring and overheating. Cernan attempted but failed to test an Air ForceAstronaut Maneuvering Unit which included a self-contained oxygen system.
On November 13, 1966,Edwin "Buzz" Aldrin became the first to successfully work in space without tiring duringGemini XII, the last Gemini mission. Aldrin worked outside the spacecraft for 2 hours and 6 minutes, in addition to two stand-up EVAs in the spacecraft hatch for an additional 3 hours and 24 minutes. Aldrin's interest inscuba diving inspired the use ofunderwater 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, Soviet cosmonautsAleksei Yeliseyev andYevgeny Khrunov transferred fromSoyuz 5 toSoyuz 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.[6]
American astronautsNeil Armstrong andBuzz Aldrin performed the first EVA on the lunar surface on July 21, 1969 (UTC), after landing theirApollo 11Lunar Module spacecraft. This first Moon walk, using self-containedportable life support systems, lasted 2 hours and 36 minutes. A total of fifteen Moon walks were performed among six Apollo crews, includingCharles "Pete" Conrad,Alan Bean,Alan Shepard,Edgar Mitchell,David Scott,James Irwin,John Young,Charles Duke,Eugene Cernan, andHarrison "Jack" Schmitt. Cernan was the last Apollo astronaut to step off the surface of the Moon.[6]
Apollo 15command module pilotAl Worden made an EVA on August 5, 1971, on the return trip from the Moon, to retrieve a film and data recording canister from the service module. He was assisted by Lunar Module Pilot James Irwin standing up in the Command Module hatch. This procedure was repeated byKen Mattingly and Charles Duke onApollo 16, and byRonald Evans and Harrison Schmitt onApollo 17.[6]
The first EVA repairs of a spacecraft were made byCharles "Pete" Conrad,Joseph Kerwin, andPaul J. Weitz on May 26, June 7, and June 19, 1973, on theSkylab 2 mission. They rescued the functionality of the launch-damagedSkylabspace station by freeing a stucksolar panel, deploying a solar heating shield, and freeing a stuck circuit breaker relay. The Skylab 2 crew made three EVAs, and a total of ten EVAs were made by the three Skylab crews.[6] They found that activities in weightlessness required about 21⁄2 times longer than on Earth because many astronauts sufferedspacesickness early in their flights.[9]
After Skylab, no more EVAs were made by the United States until the advent of theSpace Shuttle program in the early 1980s. In this period, the Soviets resumed EVAs, making four from theSalyut 6 andSalyut 7 space stations between December 20, 1977, and July 30, 1982.[6]
When the United States resumed EVAs on April 7, 1983, astronauts started using anExtravehicular Mobility Unit (EMU) for self-contained life support independent of the spacecraft. STS-6 was the first Space Shuttle mission during which a spacewalk was conducted. Also, for the first time, American astronauts used an airlock to enter and exit the spacecraft like the Soviets. Accordingly, the American definition of EVA start time was redefined to when the astronaut switches the EMU to battery power.[10]
Numerous EVAs were conducted during the assembly of theISS, often using theQuest Joint Airlock, designed to support both U.S. EMUs, and Russian Orlan space suits.
China became the third country to independently carry out an EVA on September 27, 2008, during theShenzhou 7 mission. Chinese taikonautZhai Zhigang completed a 22-minute spacewalk wearing the Chinese-developedFeitian space suit, with taikonautLiu Boming wearing the Russian-derivedOrlan space suit assisting him in the process. Zhai completely exited the craft, while Liu stood by at the airlock, straddling the portal.
Since 2021, China has carried out several more EVAs lasting several hours for the construction of theTiangong space station. In December 2024 China'sCai Xuzhe andSong Lingdong set the current record for the longest EVA at 9 hours and six minutes.[11][12]
American companySpaceX conducted the first private sector-financed EVA on September 12, 2024. EntrepreneurJared Isaacman and SpaceX engineerSarah Gillis briefly ventured outside aDragon capsule, for a stand-up EVA (SEVA) during thePolaris Dawn mission to conductspacesuit mobility testing.[13] The other two crew members were exposed to the vacuum of space in the capsule, but did not leave it.[14] SpaceX plans to launch at least two more missions involving an EVA,[15] including one that involves SpaceX's still-in-developmentStarship launch vehicle.[16]
The first spacewalk, made by Soviet cosmonaut Alexei Leonov, was commemorated in 1965 with several Eastern Bloc stamps (seeAlexei Leonov#Stamps). Since the Soviet Union did not publish details of the Voskhod spacecraft at the time, the spaceship depiction in the stamps was purely fictional.
TheU.S. Post Office issued a postage stamp in 1967 commemoratingEd White's first American spacewalk. The engraved image has an accurate depiction of theGemini IV spacecraft and White'sspace suit.[31]
 U.S.S.R. commemorative issue of 1965 |  U.S. Commemorative Issue of 1967 |
NASA "spacewalkers" during the Space Shuttle program were designated as EV-1, EV-2, EV-3 and EV-4 (assigned to mission specialists for each mission, if applicable).[32][33]
For EVAs from theInternational Space Station, NASA employed acamp-out procedure to reduce the risk of decompression sickness.[34] This was first tested by the Expedition 12 crew. During a camp-out, astronauts sleep overnight in theairlock prior to an EVA, lowering the air pressure to 10.2 psi (70 kPa), compared to the normal station pressure of 14.7 psi (101 kPa).[34] Spending a night at the lower air pressure helps flush nitrogen from the body, thereby preventing "the bends".[35][36] More recently astronauts have been using the In-Suit Light Exercise protocol rather than camp-out to prevent decompression sickness.[37][38]
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