The US space shuttle Discovery's crew awoke on Tuesday to begin unloading tons of equipment to the International Space Station, with two astronauts preparing to set out on the mission's first spacewalk.

A total of 13 astronauts at the linked shuttle and orbiting station were to help transfer cargo after Discovery's crew opened the hatch into the Leonardo Multi-Purpose Logistics Module, which was built by the Italian space agency, NASA said.

The huge pressurized chamber is carrying 7.5 tons of supplies, including new station crew quarters, a freezer, food, two research racks and a treadmill named after popular US talkshow comedian Stephen Colbert.

Astronaut Nicole Stott, the newest ISS resident, will conduct the mission's first spacewalk at 9.49pm with Mission Specialist Danny Olivas, said the US space agency.

Stott is taking over at the ISS from engineer Tim Kopra, who has been aboard the orbiting laboratory since July and is returning to Earth with the shuttle.

Discovery docked with the ISS on Sunday, on the eve of launching eight days of joint operations between the combined shuttle and station's crews.

The shuttle blasted off from Cape Canaveral, Florida, Friday after its launch was delayed three times by bad weather and a valve problem.

Astronauts are scheduled to conduct three spacewalks of six-and-a-half hours each during the 13-day mission, the fourth of five planned for the shuttle program this year. The last is scheduled for November.

One of the key goals of the spacewalks is to replace an old liquid ammonia coolant tank with a new 1760-pound unit.

The astronauts will also be retrieving experiment equipment from the exterior of the ISS and returning it to Earth for processing.