By Jonathan Amos
Science correspondent, BBC News
**The US shuttle Endeavour is preparing to lift off from the Kennedy Space Center in Florida.**The pre-dawn launch on Sunday will be the last time the orbiter climbs into dark skies.
Just five shuttle flights remain before the re-useable fleet is retired at the end of the year.
The mission to the International Space Station (ISS) will deliver a connecting unit and a large bay window that will be used as a robotic control room.
The launch is timed for 0439 local time (0939 GMT). Weather forecasters say there is an 80% chance of favourable conditions.
NODE 3 - ‘TRANQUILITY’
- Key unit connects and helps manage other ISS modules
- Multiple docking ports for visiting vehicles or future modules
- 7m by 4.6m; a mass of 14 tonnes, but will be fitted out in orbit
- Sophisticated life support systems will include air cleaning unit
- Cupola to be fixed to an Earth-facing port once in orbit
- Panoramic views provide ideal control room for robotic arm
- Named after Sea of Tranquility, the Apollo 11 landing site
The shuttle mission is the first since President Barack Obama announced a new vision for US space exploration.
Last Monday, he cancelled the rockets and capsule Nasa was developing to replace the shuttle, and urged the commercial sector to provide future transport needs.
Endeavour’s mission, which includes three spacewalks, will end construction on the Western part of the space station.
Once installed, the Node 3 and Cupola modules will make the platform 90% complete.
The mission is an important moment for the European Space Agency’s (Esa) contribution to the ISS project.
Both modules being ferried to orbit were constructed in Italy by Thales Alenia Space.
Their production concludes a barter arrangement made between Esa and Nasa in which Europe agreed to supply significant components for the platform in return for a free trip into space for its Columbus science laboratory and supporting equipment.
Some 7m in length and about 4.5m in width, Node 3 is built around the same design principles as Europe’s other space station contributions.
Columbus, Node 2, the ATV space freighter, and the Multi-Purpose Logistics Modules (which serve as the packing boxes for major re-supply missions carried out by the US shuttle) all have a similar cylindrical look about them.
Node 3 has several bays inside its multi-layer, meteoroid impact-hardened shell.
These bays will quickly become filled in orbit by equipment already on the station.
Chief among these will be the Environmental Control and Life Support Systems (ECLSS).
Their jobs involve scrubbing the air of carbon dioxide to maintain its oxygen concentration; and recycling waste water, including urine, so it can be drunk again and again.
Node 3 will also store a treadmill the crew will need to use regularly to exercise their bodies and maintain bone density.
The node has several berthing ports that could conceivably even allow the ISS to be expanded one day.
“Node 3 is an interconnecting module,” explained Simonetta Di Pippo, Esa’s director of human spaceflight.
“It’s a door open to the future, because if we decide to develop new modules, new extensions, new capabilities, we will be able to do it because we are now launching Node 3.”
The Cupola is a dome-shaped module with seven windows. At 80cm in diameter, its top window will be the biggest ever flown in space.
The module will act as a control room to direct robotic operations on the exterior of the platform, and provide a vantage point for the astronauts to view their home planet.
UK-born astronaut Nicholas Patrick will be one of the spacewalkers who will help install the modules.
“I will be undoing the bolts that hold the Cupola shutters down,” he said.
“Once those 21 bolts are released, one of my colleagues on the inside can open up the window shutters and take a first look out through the Cupola windows, which will be, I think, just a fantastic thing on the space station,” he told BBC News.
This will be the 130th space shuttle flight, the 24th flight for shuttle Endeavour and the 32nd flight to station.
Nasa hopes to run out the final four shuttle flights before the end of the year, although President Obama has promised the agency funding to support the schedule should it slip into 2011.
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