Science Fair Project Encyclopedia
Extra-vehicular activity (EVA) is work done by an astronaut away from the Earth and outside of his or her 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 the command module pilot did an EVA to retrieve film canisters on the return trip.
Due to the different designs of the early spacecraft, the American and Russian space programs also define an EVA differently. Russian, and Soviet, spacecraft have always included an airlock and consequently Russians define an EVA as occurring when a cosmonaut is in a vacuum. Early United States spacecraft, however, did not include an airlock but instead depressurized the entire spacecraft. An American astronaut was consequently not considered to have made an EVA until at least his head was outside the spacecraft. The term stand-up EVA (SEVA) is used for being partly outside.
EVAs may be tethered (the astronaut is connected to the spacecraft, oxygen can be supplied through a tube, no propulsion is needed to return to the spacecraft) or untethered. When the tether performs life support functions such as providing oxygen, it is called an umbilical. For untethered EVAs during space flight, capability of returning to the spacecraft is essential; see Manned maneuvering unit.
The first EVA was carried out by Soviet cosmonaut Aleksei Leonov on March 18, 1965 from the Voskhod 2 spacecraft. The first woman to perform an EVA was Cosmonaut Svetlana Savitskaya on July 25, 1984 while she was aboard the Salyut 7 space station.
The first EVA by an American astronaut was made on 1965 June 3 by Edward White during the Gemini 4 mission. The first American woman to make an EVA was Kathryn D. Sullivan, who stepped into space on 1984 October 11 during Space Shuttle Challenger mission STS-41-G.
The first EVA that was a moonwalk rather than a spacewalk was made by American astronaut Neil Armstrong on 1969 July 20 when the Apollo 11 Lunar Module Eagle landed on the Moon. He was joined by crewmate Buzz Aldrin, and their EVA lasted 2 hours and 32 minutes.
The first untethered spacewalk was by United States astronaut Bruce McCandless on 1984 February 7, during Challenger mission 41-B. He was subsequently joined by astronaut Robert L. Stewart during the 5 hour 55 minute spacewalk.
An EVA is dangerous for a number of different reasons. The primary one is collision with space debris. Orbital velocity at 300 km above the Earth (typical for a Space Shuttle mission) is 7.7 km/s. This is 10 times the speed of a bullet, so the kinetic energy of a small particle (e.g. a fleck of paint or a grain of sand) is equal to that of a bullet with a mass that is 100 times as large. Every space mission creates more orbiting debris, so this problem will continue to become worse.
Another reason for danger is that external environments in space are harder to simulate before the mission and that space walks are avoided for routine tasks because of their danger. As a result the EVAs are often planned late in the project development when problems are discovered, or sometimes even during an operational mission. The exceptional danger involved in EVAs inevitably leads to emotional pressures on astronauts who, though selected for exceptional stability under pressure, are still human.
Scientists are developing tele-operated robots for outside construction work, to eliminate the need for EVAs.
- NASA JSC Oral History Project Walking to Olympus: An EVA Chronology PDF document.
- NASDA Online Space Notes
- Apollo Extravehicular mobility unit. Volume 1: System description - 1971 (PDF document)
- Apollo Extravehicular mobility unit. Volume 2: Operational procedures - 1971 (PDF document)
- Skylab Extravehicular Activity Development Report - 1974 (PDF document)
- Analysis of the Space Shuttle Extravehicular Mobility Unit - 1986 (PDF document)
- NASA Space Shuttle EVA tools and equipment reference book - 1993 (PDF document)
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