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Astronauts are making their third and final spacewalk of the current shuttle mission to the International Space Station. American Robert Behnken and UK-born Nicholas Patrick are on a 6.5-hour outing to put the finishing touches on the recently-fitted Tranquility Node. Among other tasks, they will prepare the module's new observation deck, or "cupola", for its first opening. Tranquility represents the last major component in the building of the ISS. Besides a good deal more space, Tranquility will provide ISS astronauts with an unprecedented view of the Earth and of approaching vehicles that the station has lacked until now. The seven-windowed cupola includes a central window of 80cm diameter - the largest ever sent into space. The Tranquility Node houses a range of life-support systems, an exercise machine, a toilet and a pair of workstations for manoeuvring robotic equipment outside the station. The node was first installed on Friday and the first two spacewalks were undertaken on Saturday and Sunday. During those operations, Tranquility's cooling and electrical systems were connected. The ISS's robotic arm was used to relocate the cupola from the "far side" of the space station, where it had been placed for launch, to the Earth-facing side. Tuesday night's spacewalk will finish off the commissioning of Tranquility, removing heater cables that are no longer needed, connecting a secondary cooling system and installing six handrails outside the node for the benefit of future spacewalks. The cupola's thermal blanket will be removed in two pieces and stowed and the "launch locks" that held its shutters in place against the forces of launch will be removed. With the node fully up and running and the shutters free to move, the first views of Earth through the cupola could come as early as 0530 GMT when the shutters are first tested. Space shuttle Endeavour will un-dock from the ISS on Friday and is expected to land back in Florida on Sunday. Four more shuttle missions will be undertaken over the next seven months to deliver further equipment to the ISS, after which the venerable shuttle fleet will be retired.