extra

Extra-vehicular activity

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 did an EVA to retrieve film canisters on the return trip; he was assisted by the lunar module pilot who would perform a stand up EVA. These trans-Earth EVAs were the only spacewalks ever conducted in deep space to date.

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. 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). The latter is a safety device worn on tethered US EVAs, since the capability of returning to the spacecraft is essential. As of 2008, Russia, the United States and China are the only countries with the capability to conduct an EVA.

EVA milestones

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.

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 the Hydro-lab 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 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.

Alexey Leonov's EVA did not pass smoothly either. 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 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 much effort, climb back inside.

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. 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".

2008 EVAs

This is a list of EVAs conducted or scheduled to be conducted in 2008.

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).
STS-122
EVA 1
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
EVA 2
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
EVA 3
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.
STS-123
EVA 1
Richard M. Linnehan
Garrett E. Reisman
March 14, 2008
01:18
March 14, 2008
08:19
7 hours, 1 minute Installation of ELM-PS and Dextre assembly.
STS-123
EVA 2
Linnehan
Michael J. Foreman
March 15, 2008
23:49
March 16, 2008
06:57
7 hours, 8 minutes Dextre assembly (cont.).
STS-123
EVA 3
Linnehan
Robert L. Behnken
March 17, 2008
22:51
March 18, 2008
05:44
6 hours, 53 minutes Dextre assembly (cont.), prepare the Spacelab Logistics Pallet for landing, transfer a spare Canadarm2 yaw join, transfer two spare Direct Current Switching Units Mission and unsuccessful attempted installation of the MISSE 6 experiment on the exterior of the Columbus module.
STS-123
EVA 4
Behnken
Foreman
March 20, 2008
22:04
March 21, 2008
04:28
6 hours, 24 minutes Replacement of Remote Power Control Module and test of tile repair material. Removal of a sock covering the left hand of Dextre and some launch locks on Harmony. Release launch locks on Harmony’s port and nadir Common Berthing Mechanisms
STS-123
EVA 5
Behnken
Foreman
March 22, 2008
20:34
March 23, 2008
02:36
6 hours, 02 minutes Storage of Shuttle Orbiter Boom Sensor System on the Station, installation of ELM‐PS trunnion covers, removed five covers from the starboard SARJ and performed inspections, captured digital photography, successful installation of the MISSE 6 experiment on the exterior of the Columbus module and debris collection.
STS-124
EVA 1
Ronald J. Garan, Jr.
Michael E. Fossum
June 3, 2008
16:22
June 3, 2008
23:10
6 hours, 48 minutes Release straps on shuttle robotic arm elbow joint camera. Transfer OBSS back to the shuttle. Prepare Kibō for installation. Replace trundle bearing assembly on the starboard solar alpha rotary joint (SARJ). Inspect damage to SARJ. Test cleaning methods for use on the SARJ race ring.
STS-124
EVA 2
Garan
Fossum
2008-06-05
15:04
2008-06-05
22:15
7 hours, 11 minutes Installation of covers and external television equipment on the JEM and preparation of RMS. Preparation for relocation of ELM-PS. Prepare a depleted nitrogen tank assembly for removal and a new one stowed on an external stowage platform for installation. Remove television camera with a failed power supply.
STS-124
EVA 3
Garan
Fossum
2008-06-08
13:55
2008-06-08
20:28
6 hr 33 min Remove and replace starboard nitrogen tank assembly. Finish outfitting of Kibō laboratory. Reinstall removed television camera with repaired power supply.
Expedition 17
EVA 1
Sergei Volkov
Oleg Kononenko
2008-07-10
18:48
2008-07-11
1:06
6 hr 18 min Remove pyrotechnic bolt from docked Soyuz
Expedition 17
EVA 2
Volkov
Kononenko
2008-07-15
17:08
2008-07-15
23:02
5 hr 54 min Installation of a docking target on the Zvezda service module. Installation of Vsplesk experiment. Straightening of ham radio antenna. Retrieval of Biorisk experiment.
Shenzhou 7 Zhai Zhigang,
Liu Boming (stand up only)
2008-09-27
08:38
2008-09-27
09:00
00 hr 22 min First EVA conducted by China

See also

References

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