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The weather is looking good here at the for Sunday's pre-dawn launch of the Endeavour shuttle.

We had torrential rain and tornado warnings on Friday, but the forecasters say there is an 80% chance of acceptable flight conditions come 0439 EST on Sunday.

The weekend's launch is an important moment for Europe and its involvement in the space station project.

The Endeavour orbiter will deliver the final planned connecting unit, known as or Tranquility, and a dome shaped element with windows referred to as the .

Both units were made in Italy by . Thales has now produced about a half of the total pressurised volume on the US side of the platform.

I'm sure most people probably don't realise quite how much of the station's structure has been fabricated in Europe.

Much of it is the result of clever barter agreements signed between the US and European space agencies (Nasa and Esa) in the late 1990s.

Without a means to deliver its to the ISS, Europe agreed to make Nodes 2 and 3 for the outpost for "free". In return, Nasa waived the cost of the February 2008 shuttle launch that transported Columbus to orbit.

It was a smart deal because the cash that would otherwise simply just have been given to the Americans was instead invested in European industry, advancing its expertise in human spaceflight engineering.

Tranquility is a very sophisticated module and once on station will house its core life-support systems.

Its delivery, though, is being somewhat overshadowed by the Cupola, which contains the biggest window (at 80cm diameter) ever sent into space.

When you talk to astronauts, the Cupola is the one module they all say they want to see installed. And yet, this hardware has had to struggle to find itself a slot in the station assembly sequence. Indeed, it was abandoned twice.

The first time was before ISS construction had even begun, when the design of the station was scaled back and it was decided the domed window unit would not be a priority.

The module's manufacture was subsequently picked up by the Europeans under another of those barter deals, but the Cupola then found itself without a ride.

Its launch slot had been dropped as Nasa, reeling from the Columbia accident, re-scoped the station architecture again and reserved the final space shuttle flights for those ISS components deemed absolutely necessary.

From 2004 onwards, the Cupola sat in the corner here at Kennedy's . The museum beckoned.

But in the background, the astronauts were pushing to have the Cupola re-instated. And then Philippe Deloo, Esa's project manager for the Nodes and Cupola, cracked a joke:

"Offline in a small group of Nasa counterparts, I made the joke that we should fly the Cupola attached to Node 3. At the time, this was for me a joke because I didn't really believe in it. I don't know if that's what triggered the decision but basically a request from Nasa came back in 2006/7 for us to investigate the feasibility of doing it."

It's probably the best joke Philippe Deloo will ever tell. The Cupola will travel to orbit on Sunday attached to the front, or end cone, of Tranquility.

It wasn't designed to do that. It was designed to launch on a shuttle pallet. Engineers had to check the new method of carriage was possible. They've had to put in place some contingencies, but the bottom line is that the dome flies.

Once on station, it can be re-positioned to a hatch that has the correct connections to support its functions. This is on a downward facing port. When all the shutters are open, astronauts will be able to look along the full length of the station's belly.

And, of course, the Cupola will also offer a jaw-dropping view of Earth. Simonetta Di Pippo is Esa's director of human spaceflight:

"I had some astronauts who are now retired telling me: 'We would love to fly again just to look outside from the Cupola'. You know, when you spend so much time in orbit, far away from your family, far away from the Earth - it's important that you can look outside. Psychology, it's important. Talking to the astronauts, for them the Cupola is a big step."

The shutters won't be left open all the time. In fact, most of the time they will be closed. This will ensure the glass maintains its clarity and integrity for as long as possible.

These are thick windows, the 80cm-diameter one in particular. Philippe Deloo explains the construction:

There are four panes of glass. In the middle, there are two pressure panes. One is the primary pane; the other is redundant - it is there in case the first one fails so you still have a barrier to maintain the pressure. Both of these panes are 3cm thick.
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And then there are two thinner panes just over a centimetre thick. On the outside is the debris pane. It's there to protect the structural glass from micrometeorites, crew kick loads, tools, or whatever might be floating around. And then you have the same thing on the inside to protect the structural glass from the crew.

And if you're wondering: yes, they can be replaced if needed. That would be some glazing job, though.

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