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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 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 Soviet space programs also define an EVA differently. Russians define an EVA as occurring when a cosmonaut is in a vacuum. An American astronaut EVA begins when the astronaut switches the Extravehicular Mobility Unit (EMU) to battery power. The term stand-up EVA (SEVA) is used for being partly outside.
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. For untethered EVAs during space flight, capability of returning to the spacecraft is essential; see Manned Maneuvering Unit (MMU) and Simplified Aid for EVA Rescue (SAFER).
EVA milestones
- The first EVA that was a moonwalk rather than a spacewalk was made by American astronaut Neil Armstrong on July 20 1969 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 (and only) three-person EVA was performed on May 13, 1992, as the third EVA of STS-49, the maiden flight of Endeavour.[1] Pierre Thuot, Richard Hieb, and Thomas Akers conducted the EVA to capture and repair a non-functional Intelsat VI-F3 satellite, and equipped the satellite with a new motor, that after release, moved it into its planned geosynchronous orbit.
- The longest EVA was 8 hours and 56 minutes, performed by Susan J. Helms on March 11, 2001.[2]
- Anatoly Solovyev, with 16 spacewalks, totaling 82 hours, 22 minutes EVA, holds the record for most spacewalks (16), and total duration (82 hours, 22 minutes).[2]
- Captain Michael Lopez-Alegria holds the all time American record for number of EVA's (10), and for total EVA duration (67 hours and 40 minutes).[2]
EVA hazards
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 with a mass 1/100th that of a bullet (e.g. a fleck of paint or a grain of sand) is equal to that of a bullet. Every space mission creates more orbiting debris, so this problem will continue to worsen (see also Kessler Syndrome).
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 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 is undisclosed, but it was either Jerry L. Ross or Jay Apt). However, the puncturing object held in place, resulting in no detectable depressurization. In fact, the puncture was not noticed until after the space walkers were safely back inside Atlantis.[3]
Alexey Leonov's EVA did not pass smoothly, although this was not reported at the time. During the EVA, Leonov's suit had become overinflated to the point he could no longer re-enter and seal the door of the airlock. Because he was breathing pure oxygen, he was able to reduce his suit pressure to under 4 psi (28 kPa) and, with much effort, climb back inside.[4]
As of 2008, no catastrophic incident has ever occurred during an extra-vehicular activity, and no astronaut or cosmonaut has ever died during one. However, 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 potentially eliminate or reduce the need for human EVAs.
Camp out
For EVAs from the space station, NASA now routinely employs a camp out procedure to reduce the risk of decompression sickness.[5] 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).[5] Spending a night at the lower air pressure helps flush nitrogen from the body, thereby preventing "the bends."[6][7]
2008 EVAs
This is a list of EVAs conducted or scheduled to be conducted in 2008.
- See also: List of spacewalks and moonwalks and List of ISS spacewalks
| Mission |
Spacewalkers |
Start (UTC) |
End (UTC) |
Duration |
Notes |
Expedition 16
EVA 5 |
Peggy Whitson
Daniel M. Tani |
January 30, 2008
09:56 |
January 30, 2008
17:06 |
7 hours, 10 minutes |
Replaced the Bearing Motor Roll Ring Module (BMRRM) on one of the station's solar wings, inspection of Solar Alpha Rotary Joint (SARJ).[8] |
|
| STS-122 |
Rex J. Walheim
Stanley G. Love |
February 11, 2008
14:13 |
February 11, 2008
22:11 |
7 hours, 58 minutes |
Columbus power data grapple fixture installation, P1 truss nitrogen (N2) tank assembly preparation, power and data grapple fixture removal and installation to Columbus. |
| STS-122 |
Rex J. Walheim
Hans Schlegel |
February 13, 2008
14:27 |
February 13, 2008
21:12 |
6 hours, 45 minutes |
P1 Truss Nitrogen (N2) tank assembly installation, stowage of old N2 tank assembly into payload bay, Station-to-Shuttle Power Transfer System (SSPTS) routing. |
| STS-122 |
Rex J. Walheim
Stanley G. Love |
February 15, 2008
13:07 |
February 15, 2008
20:32 |
7 hours, 25 minutes |
Installation of SOLAR telescope, and EuTEF facility onto an External Stowage Platform (ESP) on Columbus, retrieval of failed Control Moment Gyroscope (CMG) that was replaced on STS-118 and stowed on ESP2, installation of failed CMG into payload bay, installation of keel pin cloth covers on Columbus. |
Planned EVAs:
| Mission |
Spacewalkers |
Start (UTC) |
End (UTC) |
Duration |
Notes |
See also
References
External links
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