Working Aloft |
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"The bravest are surely those who have the clearest vision of what is before them, glory and danger alike, and yet notwithstanding, go out to meet it." -Pericles An overhead crane swings a 10-ton building block into position. Workers climb on to the structure, and using hand and power tools, bolt the pieces together. It is a scene that could be found on almost any city street corner in America, but this construction site is 250 miles up in space, where the conditions alternate hourly between freezing and searing and from daylight to night. The construction workers are astronauts, the cranes are a new generation of space robots and the skyscraper they are building is the International Space Station. To build the International Space Station, Earth orbit will serve as a construction site for the next 5 years. Astronauts will perform more spacewalks over that time than have been conducted since spaceflight began, more than two and a half times as many. Over 150 spacewalks will be needed to complete the job. The astronauts will be assisted by the inch-worming robotic arm, the two-fingered Canada hand, and a free-flying robotic eye camera that can circle and inspect the station. Before the station's assembly is completed, more than 100 different components launched on 46 spaceflights will have been bolted, latched, wired, plumbed and fastened together to build the 1,000,000 pound station. In building and operating the station, we will gain the experience we need for future travels beyond Earth orbit to the Moon and on to Mars. Click here to manipulate a virtual reality-model of the completed ISS.
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Let’s start by listening to the astronauts who have gone on spacewalks to build the ISS talk about their job. You can just read the text of their interviews or click on the pictures to watch the video interviews. |
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Then we’ll come back and explore space suits and spacewalks in more detail.
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Space SuitsTo explore and work in space, human beings must take their environment with them because there is no atmospheric pressure and no oxygen to sustain life in the vacuum of space. Inside the spacecraft, atmosphere can be controlled so that special clothing isn't needed; but when outside, humans need the protection of a spacesuit. |
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Earth's atmosphere is 20% oxygen and 80% nitrogen from sea level to about 75 miles up, where space begins. At 18,000 feet, the atmosphere is half as dense as it is on the ground; and at altitudes above 40,000 feet, air is so thin and the amount of oxygen so small that pressure oxygen masks no longer do the job. |
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Above the 63,000-foot threshold, humans must wear space suits that supply oxygen for breathing and that maintain a pressure around the body to keep body fluids in the liquid state. At this altitude, the total air pressure is no longer sufficient to keep body fluids from boiling. Space suits for the space shuttle are pressurized at 4.3 pounds per square inch (psi), but because the gas in the suit is 100% oxygen instead of 20%, the person in a space suit actually has more oxygen to breathe than is available at an altitude of 10,000 feet or even at sea level without the space suit. Before leaving the space shuttle to perform tasks in space, an astronaut has to spend several hours breathing pure oxygen. This procedure is necessary to remove nitrogen dissolved in body fluids, and thereby, to prevent its release as gas bubbles when pressure is reduced, a condition commonly called "the bends." |
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Space suits designed for living aboard the space station will be pressurized to 8.3 psi; therefore, the prebreathing period will be shortened or diminished. The space suit also shields the astronaut from deadly hazards. Besides providing protection from bombardment by micrometeoroids, the space suit insulates the wearer from the temperature extremes of space. |
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Without the Earth's atmosphere to filter the sunlight, the side of the suit facing the Sun may be heated to a temperature as high as 250oF; the other side, exposed to darkness of deep space, may get as cold as –250oF. |
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Working AloftBecause of the unprecedented complexity of assembling so many pieces in orbit, NASA expects to encounter some surprises during the orbital construction work. But to prepare for the challenges, engineers and astronauts have been methodically practicing procedures, preparing tools, testing equipment, and building experience during more than a decade of spacewalking flight tests. A total of at least 37 space shuttle missions will assemble, outfit, and begin research use of the station from 1998 to 2006. About 160 spacewalks (or EVAs, an acronym from the technical term “extravehicular activities”) totaling 1,920 hours, will be performed during that time to assemble and maintain the station. Astronauts have spent only about 700 hours spacewalking outside the shuttle. Even adding outings by Gemini, Apollo, and Skylab crews NASA's cumulative spacewalk experience falls short of what will be expected from space station assembly and maintenance crews in the future. |
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"We have a chart that shows the buildup of the time over the next 5 years. It shows what we did starting back in Gemini and then Apollo, walking on the moon. Those are little bumps down at the bottom and then you get up to space station assembly and there's this huge spike over the next 5 years. |
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“It is really a big concern and it's something that requires constant vigilance," said Greg Harbaugh, an astronaut who heads NASA's EVA office at the Johnson Space Center in Houston. |
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EVA astronauts work in team and supported by the flight crew and Mission Control. Many of the tasks that the ISS assembly teams are doing involve connecting electrical connections between modules. However the station's power cannot be completely shut down when it's time to make the electrical connections. |
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"It would be nice to just go hit a master circuit breaker and turn everything off. But that would shut down the entire space station, so we can't do that," said astronaut William McArthur, who was paired with Leroy Chiao for two of the spacewalks on STS-92. "We've had to pay a lot of attention to how the sequence of tasks is choreographed so that we have the right plugs turned off when we stick the connectors in. It's quite an interesting task to get this all scheduled and choreographed," said McArthur. |
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Few aspects of ISS assembly tasks are easy. For example, on the STS-92 flight, the robot arm operator Koichi Wakata had to pick up and attach large pieces of equipment without a direct line of sight out the shuttle's window. |
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Instead, astronauts such as Wakata rely on an artificial space vision system developed by the Canadian Space Agency, and the directions of spacewalkers positioned outside the shuttle. The margin for error is slim. If a crew fails to attach a new component correctly for some reason, the station assembly sequence can come to a halt. "Every mission has to work because the follow-on missions depend on you getting your things done," said the STS-92 Discovery commander Brian Duffy. "That fact hasn't been lost on my crew nor on all of the other crews that are assigned and training right now."
Space Station Extravehicular Activities |
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Astronaut Carl Walz, a member of the fourth crew that will live aboard the International Space Station, tries on a Soyuz space suit |
Before the arrival of the Joint Airlock Module on shuttle mission STS-104, spacewalks conducted from the space station will only use Russian space suits, unless the space shuttle is present. The Russian Service Module provides a capability for station-based Russian spacewalks using only Russian space suits. The Joint Airlock Module, which will be attached to the station during the ninth space shuttle assembly flight, will give the station the capability to conduct spacewalks using U.S. space suits. |
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The Joint Airlock Module, which has the capability to be used by both Russian and U.S. space suit designs, consists of two sections, a "crew lock" that is used to exit the station and begin a spacewalk, and an "equipment lock" used for storing gear. |
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The equipment lock also will be used for overnight "campouts" by the crew, during which time the pressure in the Joint Airlock Module is lowered to 10.2 pounds per square inch (psi) while the rest of the station remains at the normal sea level atmospheric pressure of 14.7 psi. The night spent at 10.2 psi in the Airlock purges nitrogen from the spacewalkers' bodies and prevents decompression sickness, commonly called "the bends," when they go to the 4.3 psi pure oxygen atmosphere of a space suit. Station crewmembers could perform a spacewalk directly from the 14.7 psi cabin atmosphere, but they would have to go through a several hours-long prebreathe of pure oxygen first. |
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The Airlock "campout" shortens the pure oxygen prebreathe time to only minutes for the crew. The protocol is similar to a procedure commonly used in advance of space shuttle spacewalks, in which the shuttle's cabin pressure is lowered to 10.2 psi at least a day ahead of the EVA. After the Joint Airlock Module is operational, the philosophy of spacewalk training will shift due to the increasing complexity of the station and the ability of the station crew to perform spacewalks. |
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Rather than attempting to train station crewmembers for every EVA task they may be called upon to perform during a mission, training will increasingly aim toward providing crew members with a general suite of EVA skills. The station's growing size and complexity will make it virtually impossible for astronauts to train for every possible contingency and maintenance EVA, as is the case in training for shuttle missions. Training is done on Earth in the Neutral Buoyancy Laboratory underwater tank and in virtual-reality simulators.
Tools in Space |
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NASA, in recognizing the challenge of building the International Space Station, NASA has for more than a decade been working on developing and flight-testing the equipment needed for spacewalks and refined spacewalk training procedures. |
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In order to build spacewalk experience among astronauts, engineers and flight controllers, over a dozen "practice" spacewalks have been conducted during space shuttle flights since 1991. The three servicing missions for the Hubble Space Telescope on STS-103 helped to prepare crews, trainers, ground control teams, and engineers for the tools and techniques needed to build the ISS. Many of the astronauts who gained experience during these "practice" spacewalks will bring that knowledge to bear during future spacewalks as the station's orbital assembly begins. |
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The flight-testing of EVA equipment designed for use aboard the International Space Station began on the first spacewalk NASA conducted after the space shuttle's return to flight following the Challenger accident. |
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On shuttle mission STS-37 in April 1991, Astronauts Jerry Ross and Jay Apt performed a spacewalk to test the Crew and Equipment Translation Aid cart designed for use in assisting astronauts to move about the football-field-long truss of the completed station. Two such carts are now planned for launch to the station during its assembly. Ross served as the lead spacewalker on the first station assembly mission, shuttle mission STS-88 in December 1998. Since 1991, other spacewalks have evaluated new tethers, tools, foot restraints, techniques for handling large masses, a jet pack "life jacket," space suit enhancements, and even the planned station lettering and toolboxes. To prepare for International Space Station assembly in earnest, NASA announced the first International Space Station EVA assembly crew, Ross and Jim Newman on STS-88 in August 1996. In June 1997, five more crews of station assembly spacewalkers were named to complete the first six shuttle assembly missions. Some of these crews were named more than 2 years ahead of their scheduled mission, much earlier than is traditional. The early naming of crew members has allowed the astronauts additional time to train for their complex and crucial missions. Space RoboticsTo assemble and operate the ISS astronauts on EVAs will work in tandem with a new generation of space robotics. The space shuttle's mechanical arm and a new space station arm will operate both as cranes to precisely maneuver large modules and components, and also like space cherry pickers, to maneuver astronauts to various working areas. |
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The shuttle's robotic arm uses a new space vision system that helps the arm operator see around corners. Tested on past space shuttle missions STS-74, STS-80, and STS-85, the system uses video image processing and a series of markings on the objects being maneuvered to develop a graphical laptop computer display to assist the arm operator. |
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It allows the shuttle arm to be operated with great precision even when visibility is obstructed. The system was used operationally during the first assembly mission, STS-88, as astronaut Nancy Currie, with her view partially obstructed, attached the first station component, the Zarya control module, to the second component, the Unity connecting module. |
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Canada also is building the new Space Station Remote Manipulator System, a 55-foot-long arm that will be launched early in the station's assembly sequence. |
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The station arm will have the new capability to move around the station's exterior like an inchworm, locking its free end on one of many special fixtures (called Power and Data Grapple Fixtures) placed strategically around the station, and then detaching its other end and moving forward. In addition, the station arm will later ride on a Mobile Servicing System platform that will move on tracks along the length of the station's 360-foot truss, putting much of the station within grasp of the arm. |
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Canada also is providing the new robotic "Canada Hand" for the station, called the Special Purpose Dexterious Manipulator (SPDM), scheduled to be launched in 2002. The "hand" consists of two small robotic arms that can be attached to the end of the main station arm to conduct more intricate maintenance tasks. Click here for more about the Space Station Remote Manipulator System. Two other robotic arms will be on the International Space Station. A European Robotic Arm (ERA), built by the European Space Agency will be used for maintenance on the Russian segment of the station. The Japanese laboratory module will include a Japanese robotic arm that will tend exterior experiments mounted on a "back porch" of the lab. |
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In addition to mechanical arms, other robotics that may be used aboard the station include a free-flying robotic camera, a prototype of which was tested during a 1997 space shuttle mission, that may be used to inspect the exterior of the station, including the acre of solar panels. Called the AERCam, the prototype may undergo more flight tests on future shuttle missions. |
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Click here for more about the AERCam.
The Station Suit |
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In addition to new spacewalking tools and techniques, astronauts will have an enhanced space suit. The shuttle space suit, or Extravehicular Mobility Unit (EMU) as it is technically called, was originally designed to be maintained between flights by specialists on Earth. |
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This is a difficult requirement for astronauts living and working on the space station. The EMU is now being improved for use on the ISS. Click here for details about the EMU. The space suit will be stored in orbit and will be certified for up to 25 spacewalks before it must be returned to Earth for refurbishment. It can be adjusted in flight to fit different astronauts, and it can be easily cleaned and refurbished onboard the station between spacewalks. In addition, assembly work on the station will be done in much colder temperatures than are experienced during most space shuttle spacewalks. Unlike the shuttle, the station cannot be turned to provide the optimum sunlight to moderate temperatures during a spacewalk. |
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For a detailed history of space suits, “Walking to Olympus”, click here. |
Enhancements to the suit to better prepare it for assembly and use aboard the station include: easily replaceable internal parts; reusable carbon dioxide removal cartridges; metal sizing rings that allow in-flight suit adjustments to fit different crewmembers; new gloves with enhanced dexterity; a new radio with more channels to allow up to five people to talk at one time; warmth enhancements such as fingertip heaters and a cooling system shutoff; new helmet-mounted floodlights and spot lights; and a jet-pack "life jacket" called SAFER to allow an accidentally untethered astronaut to fly back to the station in an emergency. |
Questions to think about:
In the next unit, Space Station Science you will explore the many exciting new experiments planned for the space station. Long-duration spaceflight will change the nature of science done in low-Earth orbit and will lead to many benefits for people on Earth as well as in space. Next... Mission
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