An unprecedented series of five spacewalks by two pairs of astronauts working in shifts are the highlights of the 11-day mission to repair the dollars 1.5bn Hubble space telescope.
It is costing about dollars 629m ( pounds 425m) to send the team of seven astronauts about 370 miles above Earth to rendezvous with the Hubble telescope and correct its defective mirror.
Observers of the United States National Aeronautics and Space Administration (Nasa) believe it is the space agency's most ambitious attempt at demonstrating the importance of manned space flight.
Martin Rees, Professor of Astronomy at Cambridge University, said that if the mission, which is due to launch on Wednesday, fails 'it will be deeply embarrassing for Nasa. This exercise next week is to show that putting men and women into space is worth something'.
The four astronauts who will take part in the spacewalks have a formidable array of tasks to perform, from fitting the delicate gyroscopes that keep the telescope pointing in the right direction with pinpoint accuracy to unfurling and replacing the 12-metre long solar panels.
Their most sensitive task is to fit a two-metre long box of mirrors that will correct the optical defects of the telescope's main mirror. If it works, the telescope should be able to meet its specification of resolving very distant sources of light - equivalent to distinguishing between the two rear lights of a car at more than 6,000 miles or being able to read a newspaper from more than half a mile away.
Story Musgrave, the astronaut who will take a lead role in the spacewalks, is a trained surgeon and said he will need all his skills for the delicate operations he and his colleagues have to perform.
Franco Bonacina, a spokesman for the European Space Agency, said: 'It is like working in an operating theatre while wearing the equivalent of boxing gloves.'
The complexity of the tasks the astronauts have to perform are such that Nasa has had to plan for three additional spacewalks if something goes wrong in the first five. A robot arm will be deployed to help the crew capture the telescope and lock it to the cargo crew of the Endeavour, the latest shuttle, which has the fuel and oxygen storage necessary for such a long mission.
Hubble has been dogged by problems in the 20 years of the project. Delays resulted in the initial launch date of 1983 being put back to 1990, largely because of the shuttle Challenger disaster of 1986 which killed all seven crew.Reuse content