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Once on the ground, the cylinder appears to take on a life of its own - its perfectly curved wall begins to distort as it slowly swells to almost double its original diameter. A row of space-suited figures file through an airlock from the spaceship to enter the shiny, amorphous tube. It will be the space-travellers' Martian home for their sojourn, 35 million miles from planet Earth.

This is the scenario that Foreign Office Architects is working to create. Its design for an expanding cylindrical accommodation capsule has reached the final five in a competition organised by Nasa to design a house for Mars. In May this year, the first prototype of the dwelling will be unveiled at Nasa's Mars exhibition in Malm枚, Sweden, as part of the Future Homes display.

The expanding cylinder is fundamental to the scheme's success. The challenge for the project's structural engineer, Adams Kara Taylor, was to develop a simple structure that would allow the cylinder's walls to expand outward, creating as much accommodation space as possible, without adding unnecessary weight to the rocket's payload.

"The cylinder's geometry is dictated by what can be transported to Mars," says Hanif Kara, a principal of the practice. A cylinder would fit snugly into a rocket's 4.2 m diameter, 11 m long cargo hold. "But this cylinder would not be spacious enough to provide the accommodation in which a group of scientists could live for months on end. We had to design a structure to allow the cylinder's walls to expand." Kara's solution involves a series of expanding structural hoops. These are spaced 1 m apart along the length of the cylinder so that, without its skin, the capsule looks like a whale's ribcage.

Each circular rib comprises three semicircular structural members: a fixed section and two movable top sections. The ribs' movable sections are pivoted from each end of the fixed section. During transportation, these movable sections are folded down on top of each other to form half of the cylinder's circular cross-section. The semicircular fixed base completes the circle. E E Once the capsule is unloaded, the top sections swing outwards to form the enlarged accommodation module (diagram, right).

In the prototype, steel will be used to form the cylinder's structural ribs. Later in the year, however, the engineers will visit Nasa to discuss the use of advanced composite materials. Likewise, although standard electric motors will be used to unfold the structure in Malm枚, Kara says the robotics that Nasa has are "far more advanced and could be adapted for the real thing".

The next challenge was to find a suitable material with which to clad the capsule. Mars is an inhospitable place. Once it may have been warm and wet, but today the red planet is a frozen wasteland with temperatures as low as 鈥�120潞C. Scientists say that it is highly unlikely that any living creature, even a microbe, could survive for long on the planet's surface.

Under these conditions, the performance of the capsule's cladding is critical for the occupants' survival. Not only does this skin have to be able to withstand the Martian environment, it will also have to be flexible enough to stretch to twice its surface area as the cylinder expands. And, given the difficulties of carrying out repairs in situ, it would have to be incredibly strong. Such a material has yet to be developed, so the prototype will use silvered PVCu. But Kara is optimistic that, given Nasa's resources, a suitable material can be developed soon.

Inside the prototype, a ramp will allow visitors access to the upper levels of the accommodation capsule. However, because the top sections of the structural ribs are movable, supporting the ramp is a problem. The engineer's solution has been to gradually rotate this lower section of the structural ribs along the length of the cylinder, in effect putting a twist into the rib cage. This rotation means that at one end of the cylinder the base section forms an upright U, while at the opposite end it forms a C 鈥� a process Kara describes as similar to twisting a piece of rainwater guttering through 90潞 along its length.

With this twist in place, the fixed section of the ribs can be used as structural support for the ramp, which can either be cantilevered or hung from the structure. However, if the scheme does land on Mars, the ramp will not be necessary. Mars has a gravitational force about one-third that of Earth, so instead of a ramp, the architect intends the capsule's occupants to use trampolines to bounce to the upper levels.

At the moment, the design team is racing to complete the prototype in time for the Malm枚 exhibition. But it is the next stage of the project that Kara is really looking forward to: "We'll be visiting Nasa later in the year when we'll have access to some of their engineers," he says. In the mean time, more terrestial matters occupy Kara's time: he still has to find a steelwork fabricator in Sweden to construct the structure for the prototype.