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"This is not a building solution, it's pure engineering," Kevin Martindale says, waving at a shiny, truncated copper-coloured cone that emerges from the ground at Greenwich park. This is the Peter Harrison Planetarium, centrepiece of the National Maritime Museum's Time and Space project. The project team think it might be the most extraordinary project in Europe at the moment, you could certainly make a case for it being the most difficult to construct - hence Martindale's remarks about engineering. And he should know: as Gardiner & Theobald construction manager, he is responsible for erecting it.

The planetarium resembles an alien space ship half buried in Greenwich park after a crash landing

The "copper" is in fact phosphor bronze. The cone is clad with sheets up to 12.5 m long, curved and tapered to fit the structure's form. Eighteen sheets will be welded together so that they appear to be one perfect, seamless object. The phosphor bronze sheets are a hefty 8 mm thick yet have been made to a tolerance of 卤 0.2 mm. But phosphor bronze is notoriously difficult to weld as it shrinks and distorts with heat, which could make the cone look more like a corrugated iron shed than a perfectly machined artifact. It was so difficult to find a specialist contractor special enough for this that the bronze concept was nearly abandoned. Furthermore, it sits over an inner concrete cone with such bizarre geometry that building it posed a serious challenge.

So why go for such an unbuildable building? According to Diane Haigh, Allies and Morrison's project architect, the legacy of the site demanded nothing less. This is the Greenwich Observatory after all, long associated with the feats of engineering that allowed humans to probe heaven and earth - from telescopes for observing the stars to the Harrison clocks that first helped sailors plot longitude at sea. "We wanted the visible part of the planetarium to have some resonance with the place," she says.

According to Haigh, the geometry of the cone is defined by reference to the celestial equator, the Pole Star and planetary orbits. And because it was to have the appearance of a scientific instrument, it had to have a flawless appearance. "We considered a whole range of cladding materials," says Haigh. "One solution could have been a rendered cone but the eye picks up imperfections. So that's why we went for this precision, engineered solution."

The concrete beneath the cladding is 225 mm thick, which is enough to isolate those within from the world outside. "The sense of wonder and suspension of disbelief when you are in the planetarium would be completely ruined by the noise of people outside - you would be brought down to earth with a bump."

This exacting brief presented the project team with a multitude of challenges. Just physically working on it was tricky because it tilts so steeply: one side is at right angles to the ground and the other is at 55.5掳. "If you use a cherry picker on the slanting side you still can't reach out and touch it," says Martindale. It also made setting out extremely difficult because the base was neither a circle nor an ellipse, and the whole site slopes.

The phosphor bronze sheets are made up of several smaller panels welded together in the factory

The inner cone was made by spraying concrete onto mesh fixed to a steel frame. Getting this in the right place required specialist help. "The geometry meant there was no reference point, so we had a firm of surveyors check at every stage," says Martindale. In theory, if they set out the base accurately, the pre-cut and drilled steelwork would naturally fit together. This theory didn't quite hold up in practice. "When we measured it at the top it was 40 mm out, so we pulled it into position and held it in place."

The concrete could then be sprayed and smoothed flat ready for the bronze. But it took longer than planned. "It took a long time to get the mix right," says Martindale. "And it took a lot longer than we thought to get the setting out and the tolerances right."

Fortunately the project was procured using construction management, and the flexibility this provided meant that battles over delays were avoided (see "Procuring Space and Time", below).

But the concrete was easy compared with the cladding. The basic problem was finding someone to do it. The design team's hopes rested on one firm - Tyne Tubes, who made Antony Gormley's Angel of the North sculpture. The company was hired to give advice, but declined to tender for the actual work. It seemed then that the grand vision was no more than a mirage. Sir John Tiltman, the director of the Space and Time project, says: "The tenders from the other specialists were massively over budget. At that point we seriously questioned whether we could go on with the bronze."

Luckily a firm called Responsive Engineering Group heard about the job from Tyne Tubes and approached the project team. Precision is its specialty: water jet cutting, welding, machining. They make the tooling for airliner wings. They make deep-sea pipelines and the titanium conrods for Formula 1 racing cars. Surely they could build a bronze cone! The project team visited the firm's factory in Gateshead and were thrilled: "Everything was spot on," says Martindale.

The project team in action around the extremely awkward geometry of the planetarium

The biggest challenge was welding the bronze without warping it. "We've had a lot of experience welding difficult components so we have techniques for countering that," says Peter Bernard, group managing director of Responsive. The welding took place in two stages. First, a series of smaller panels were welded together in the factory to make a big panel, and finally the big panels were welded together on site (see "Making the panels", below).

A neat system has been devised to ensure that the welds that are to be done on site are perfect. Steel bars are fixed near the weld edges. These cleverly make a lifting jig for positioning the panels. Once the panels have been positioned, a device called a turnbuckle is attached between the steel bars to bend the edges of the bronze outwards. "The skill is knowing by how much," says Bernard. "The turnbuckles preset the bronze so when you put the heat on, it returns to its perfect shape."

The welding starts on site next week and will continue for three months. Then the surface will be cleaned and made ready for the finishing touch - treating the bronze in stages to achieve a deep, complex patina.

Finally, sophisticated laser projectors will be installed inside the cone to give visitors the ultimate astronomical experience.