The largest window built for use in space has been completed, promising to give astronauts a spectacular view from the International Space Station.
The Cupola structure will be fitted to the space station
The 80cm-wide window is one of seven fitted to an observation dome called Cupola, which will be attached to the ISS in January 2009.
Cupola has six trapezoid-shaped side windows around the large, circular one.
The new dome is designed to allow astronauts to control the robotic arm on the outside of the space station.
The Cupola measures 2m in diameter and 1.5m in height and will be attached to Node-3 before the end of the decade. Node-3 is a new module designed for the ISS, and is currently still being assembled.
Each window is 10cm thick and consists of four layers of fused silica. The two middle layers resist the force of the pressurised interior. The inner layer is designed to withstand knocks from astronauts' boots and tools.
The outer layer has to protect against tumbling space debris; a tiny piece of metal from an old satellite travelling at 18km per second has far more energy than a rifle bullet.
The project was transferred to Esa from Nasa
The Cupola project was started in by the US space agency (Nasa) and aerospace company Boeing, but was cancelled during a cost-cutting round.
After a barter agreement between Nasa and the European space agency (Esa), development of the structure was taken over by Europe in 1998.
The 1.8-tonne Cupola has been built by Italian firm Alenia Spazio and will be transported to Kennedy Space Center in Florida, US, where it will undergo testing before being passed for flight.