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 American and Soviet space programs also define an EVA differently. Russians 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.
Spacewalks are dangerous for a number of different reasons. The primary one 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. This means that a particle approximately 100 times smaller than a bullet would have the same effect at that velocity. Nearly every space mission creates more orbiting debris, so this problem will continue to worsen.
Another reason for danger is that external environments in space are harder to simulate before the mission, though approximate simulations can be achieved at facilities like the Hydro-labs in Star City's Gagarin Training Center and NASA's Neutral Buoyancy Laboratory. 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.
Other possible problems include a space walker becoming separated from his or her craft or suffering a spacesuit puncture which would depressurize the suit, causing anoxia and rapid death if the space walker is not brought into a pressurized spacecraft quickly.
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 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 4 psi (28 kPa) and, with effort, climb back inside.
As of 2009, no catastrophic incident has ever occurred during an extra-vehicular activity, and no astronaut has ever died during one. Given the considerable hazards inherent in EVAs, and the resultant risk to astronauts, some scientists are working to develop tele-operated robots for outside construction work, to reduce or potentially eliminate the need for human EVAs.
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 10.2 psi (70 kPa), compared to the normal station pressure of 14.7 psi (101 kPa). Spending a night at the lower air pressure helps flush nitrogen from the body, thereby preventing "the bends".
Published - July 2009
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