The first spacewalk on the Discovery shuttle's latest mission will take place on Saturday.
The British-born astronaut Piers Sellers and colleague Mike Fossum will carry out repairs to the exterior of the International Space Station.
They will also test a boom-arm which Nasa hopes future missions could use to go under a shuttle to carry out inspections and repairs.
The spacewalk is expected to last about six and a half hours.
It will be Fossum's first Extravehicular Activity (EVA); Sellers was a spacewalker on a previous shuttle mission, in 2002.
Both men have practised the walk back on Earth, in a virtual reality simulation and in the US space agency's (Nasa) Neutral Buoyancy Laboratory.
http://newsimg.bbc.co.uk/shared/img/o.gifDISCOVERY SHUTTLE FLIGHT
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Mission known as STS-121
Discovery's 32nd flight
18th orbiter flight to ISS
Lift-off: 1438 EDT, 4 July
Location: Kennedy Space Center, Launch Pad 39B
Objective: To test new safety equipment and procedures
Payload: Cargo bay has 12.75t of equipment and supplies
Crew: Lindsey, Kelly, Fossum, Nowak, Wilson, Sellers, Reiter
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Sellers ready for 'big walk'
The latter is a giant swimming pool that goes some way to replicating the "weightless" conditions of space.
Of particular interest will be the boom tests. The shuttle's 15m-long (50ft) robot arm will be coupled to a new 15m-long inspection rod. The astronauts will bounce up and down on the extension to see if it is strong enough to serve as a work platform in case astronauts one day have to make repairs on a shuttle's belly.
"It's essentially one big stick," Sellers told the BBC News website before launch. "We're going to stand on the end of that and do a whole lot of variations."
The ISS repairs involve cabling work on a mobile transporter system on the exterior of the station.
Discovery docked with the ISS on Thursday and on Friday attached an Italian-built cargo module.
The mission will deliver almost 13 tonnes of supplies and equipment. In addition to food, clothing and other essentials, Discovery has brought a freezer for experiment samples, a European Space Agency (Esa) incubator to grow plants in space and a new oxygen generator so the station's crew size can eventually be doubled to six.
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Sellers will test a robotic arm boom extension on the flight
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http://newsimg.bbc.co.uk/nol/shared/img/v3/opennews.gifMore details
Once all the new deliveries have been safely stowed aboard the ISS, the module will be loaded with rubbish for return to Earth.
One of Discovery's most important deliveries has been an astronaut: the German Thomas Reiter. He is going to stay on the ISS when the rest of the Discovery crew returns to the ground.
Reiter's presence on the platform brings the resident crew up to three - the first time this has happened since ISS numbers were reduced in the wake of the Columbia disaster in 2003. The German becomes the first Esa astronaut to experience an extended tour on the orbital outpost.
