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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 CM hatch. These trans-Earth EVAs were the only spacewalks ever conducted in deep space.

Due to the different designs of the early spacecraft, the Americanmarker and Sovietmarker space programs also define an EVA differently. Russiansmarker define an EVA as occurring when a cosmonaut is in a vacuum. An American EVA begins when the astronaut switches the Extravehicular Mobility Unit (EMU) to battery power. 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 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. 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.

, Russiamarker, the United Statesmarker, and Chinamarker are the only countries with a demonstrated capability to conduct an EVA.


EVA hazards

Spacewalks are dangerous for a number of different reasons, and are avoided for routine tasks. Consequently, EVAs are often planned late in the project development when problems are discovered, or sometimes even during an operational mission.

The primary danger is collision with space debris. Velocity while orbiting 300 km above the Earth (typical for a space shuttle mission) is 7.7 km/s. This is approximately ten times the speed of a bullet. Nearly every space mission creates more orbiting debris, so this problem will continue to worsen. Other possible problems include an astronaut becoming separated from his or her craft or suffering a spacesuit puncture which would depressurize the suit, causing anoxia and rapid death if the astronaut is not brought into a pressurized spacecraft immediately. Another reason for danger is that external environments in space are harder to simulate before the mission, although approximate simulations can be achieved at facilities like the Hydro-labs in Star Citymarker's Gagarin Training Centermarker and NASAmarker's Neutral Buoyancy Laboratorymarker.

One astronaut has suffered a spacesuit puncture. During STS-37, a small rod punctured the glove of one of the astronauts (the name was not disclosed, but it was either Jerry L. Ross or Jay Apt). The puncturing object held in place, resulting in no detectable depressurization, and was not noticed until after the space walkers were safely back inside Atlantis. Alexey Leonov's original EVA did not pass smoothly either. During the EVA, Leonov's suit became overinflated to the point he could no longer re-enter or seal the door of the airlock on Voskhod 2. Because he was breathing pure oxygen, he was able to reduce his suit pressure to under and, with effort, climb back inside.

No catastrophic incident has ever occurred during an extra-vehicular activity, and no astronaut has ever died during one. Nevertheless, given the considerable hazards inherent in EVAs, some scientists are working to develop tele-operated robots for outside construction work, in order to reduce or potentially eliminate the need for human EVAs.

Camp out

For EVAs from the International Space Station, NASA now routinely employs a camp out procedure to reduce the risk of decompression sickness. This was first tested by the Expedition 12 crew. During a camp out, astronauts sleep overnight prior to an EVA in the airlock, and lower the air pressure to , compared to the normal station pressure of . Spending a night at the lower air pressure helps flush nitrogen from the body, thereby preventing "the bends".

EVA milestones

Capability milestones



Personal cumulative duration records



National, ethnic and gender firsts



Designations

NASA "spacewalkers" are desingated as EV-1, EV-2, EV-3 and EV-4 (assiged to Mission specialists for each mission, if applicable).

See also







References

  1. http://www.nasa.gov/centers/marshall/news/background/facts/evarm.html_prt.htm
  2. http://www.nasa.gov/centers/marshall/news/background/facts/evarm.html


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