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.

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."

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.)

"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.

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.

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."

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