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On the ground, the International Space Station would be an odd looking building -- but space is an odd place to live!

NASA
Marshall Space Flight Center

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see captionMarch 14, 2001 -- Homes on Earth provide shelter from the wind and rain. But a home in Earth orbit must shield its occupants from the solar wind, and it must withstand a steady rain of dust-sized meteoroids, many moving faster than a speeding bullet! 

A terrestrial house has insulation to keep the air inside cool or warm, but a space home must be tightly sealed just to keep the air inside.

The structure of earthbound buildings must support a constant gravitational pull of 1-g. In contrast, an orbiting structure's design should make sense in microgravity, and at the same time be able to withstand the tremendous 3-g acceleration of a rocket blasting into space.

For these and other reasons, building a structure for living in space poses a different set of design challenges than building homes on the ground.

Above: The International Space Station looks quite unlike homes built here on Earth. The unique conditions of space lead to a design that looks more like modern art made from soup cans than what most people think of as a home! [click image to enlarge]

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The first thing an architect would notice about building in space is the pull of gravity -- or rather the lack thereof! A freely-falling space home in Earth orbit can take a wider variety of basic shapes than homes on the planet below.

"It's in free fall, so there's no need to say 'this is up' and 'this is down' from the standpoint of the station's architecture and structural integrity," said Kornel Nagy, structural and mechanical systems manager for the International Space Station (ISS) at NASA's Johnson Space Center.

For example, science fiction writers often imagine that a space station would be wheel shaped. As seen in the Stanley Kubrick / Arthur C. Clarke science fiction classic 2001: A Space Odyssey, these ring-shaped outposts would slowly rotate to create a centrifugal pull that acted as a false gravity. Other visionaries, such as NASA's own Wernher von Braun, also saw a spinning wheel as the most likely space station design.

So why does the ISS look more like an Erector Set than a big hamster wheel?

"Even though (the wheel design is) an elegant concept," says Nagy, "you have to think in terms of the current launch vehicles that we have and how you get all the pieces on board and assembled into a unified body."

"So the option that was looked at is to take the pressurized compartments up in segments that are as big as you can lift in a particular launch vehicle," he continued. "In our case, it's the Shuttle payload bay."

see caption

Above: Building a home for living in space requires a little more than plywood and two-by-fours. Titanium, Kevlar, and high-grade steel are common materials in the ISS. Engineers had to use these materials to make the structure lightweight yet strong and puncture-resistant.

Because each of the aluminum-can shaped components of the Station has to be lifted into orbit, minimizing weight is crucial. Lightweight aluminum, rather than steel, comprises most of the outer shell for the modules.

This shell must also provide protection from impacts by tiny meteoroids and man-made debris. Because the ISS zips through space at about 27,000 km/h, even dust-sized grains present a considerable danger. Man-made debris, a drifting legacy of past space exploration, poses an even greater threat.

To ensure the safety of the crew, the Space Station wears a "bullet-proof vest." Layers of Kevlar, ceramic fabrics, and other advanced materials form a blanket up to 10 cm thick around each module's aluminum shell. (Kevlar is the material used in the bullet-proof vests used by police officers.)

see caption"This protective shielding was tested by shooting at it with high-velocity guns to verify that it is indeed a good protection material," Nagy said.

Left: Layers of Kevlar and other impact-resistant materials reduce the chance that small debris could penetrate the modules' walls and endanger the crew. [click image to enlarge]

Designers had to leave a few holes in this armor so the crew could occasionally enjoy the spectacular view.

A typical window for a house on Earth has 2 panes of glass, each about 1/16 inch thick. In contrast, the ISS windows each have 4 panes of glass ranging from 1/2 to 1-1/4 inches thick. An exterior aluminum shutter provides extra protection when the windows are not in use.

The glass in these windows is subject to strict quality control, because even minute flaws would increase the chance that a micro-meteoroid could cause a fracture.

In orbit, a major force is the pressure of the air inside the ISS, which presses on each square inch of the modules' interior with almost 15 pounds of force. (Homes on Earth also have this internal pressure, but the external pressure of the atmosphere balances it out.)

But even before reaching orbit these modules must also hold up to the massive stresses of launch. 

"The structure has to withstand the loading it will see while being transported to orbit, which is a pretty intense environment," Nagy said.

As the Shuttle climbs toward the edge of space, every piece of the ISS module inside will "weigh" three times normal. The structure of the modules must handle both this loading along the long axis during launch and the internal air pressure while in orbit.

see captionRight: The interior of the ISS modules falls a little short of Martha Stewart standards! Form follows function in the Space Station's utilitarian design. Notice the air-tight hatch of the Common Berthing Mechanism at the far end of the module.

Once the Shuttle has carried a module into orbit, the task remains to securely attach it to the rest of the Station.

The US-designed Common Berthing Mechanism (or CBM) links together the modules. To ensure a good seal, the CBM has an automatic latching mechanism that pulls the two modules together and tightens 16 connecting bolts with a force of 19,000 pounds each! This huge force is needed to counteract the tendency of the internal air pressure to push the modules apart and to ensure a good air-tight seal.

"A lot of development work, a lot of testing, and a lot of certification went into the CBM to be able to achieve that reliable seal," Nagy said. "So far it's worked well."

This is the first in a series of articles about the construction of the ISS. Future installments will examine the plumbing, heating/cooling, and ergonomics of the Station.

Web Links

International Space Station -- home page by NASA

Seeing the International Space Station from your own backyard -- Science@NASA article: With the aid of free NASA software you can spot the International Space Station from your own back yard.

Looking Forward to the ISS -- Science@NASA article: Scientists at a recent media forum say they are eager to begin using the International Space Station as an innovative orbiting research laboratory.

Wheels in the Sky -- Science@NASA article: The pioneering space station concepts of the mid-1950's don't look much like the erector-set habitat in orbit today.

A New Star in the Sky -- Science@NASA article: Something in the heavens is growing brighter and it will soon become one of the more eye-catching stars in the night sky. No, it's not a supernova. It's the International Space Station!

Breathing Easy on the Space Station -- Science@NASA article: Life support systems on the ISS provide oxygen, absorb carbon dioxide, and manage vaporous emissions from the astronauts themselves. It's all part of breathing easy in our new home in space.

Water on the Space Station -- Science@NASA article: Rationing and recycling will be an essential part of life on the International Space Station. In this article, Science@NASA explores where the crew will get their water and how they will (re)use it.

Microscopic Stowaways on the ISS -- Science@NASA article: Wherever humans go microbes will surely follow, and the Space Station is no exception.


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