The US space shuttle Endeavour is set to launch on the final orbiter mission of 2008, carrying cargo bound for the International Space Station (ISS).
Endeavour will take equipment to refit the ISS for six crew members, instead of the current three, and will also drop off astronaut Sandra Magnus.
Four spacewalks on the 15-day flight are planned, including repairs to joint damage on the station’s solar arrays.
The shuttle will launch from the Kennedy Space Center in Florida.
The orbiter is expected to get away from the launch pad at 0055 GMT on Saturday.
Repair and servicing of the space station solar arrays
Installation of new crew quarters and water system
Drop off astronaut Sandra Magnus and pick up Greg Chamitoff
Meteorologists predict a 70% chance the weather will be suitable for Nasa’s 124th shuttle launch.
A Nasa spokesman said: “The launch team at Nasa’s Kennedy Space Center has started filling the external tank with liquid oxygen and liquid hydrogen.
“The propellants will power Endeavour’s three main engines during launch.
“There are no technical issues in work and lift-off remains on schedule.”
The mission, STS-126, has been titled “Extreme Home Improvements”.
During the 15 days, the crew will conduct four spacewalks, to work on the space station’s solar power system.
Working in teams of two, astronauts will emerge from the space station’s Quest airlock and work on the two large rotary joints, which turn the station’s massive solar array “wings” towards the Sun.
They are to service the starboard side joint and perform preventative maintenance on the port side joint.
With regards to “home improvement”, the crew will install new equipment, including new crew quarters, with an additional bathroom, and a galley.
This includes two new sleeping compartments, more exercise gear, and a second toilet.
Nasa plans to double the station’s crew size from three to six as early as May.
Sandra Magnus (left) will stay aboard the ISS until the spring
“With six people, you really do need to have a two-bathroom house. It’s a lot more convenient and a lot more efficient,” said Endeavour crew member, Sandra Magnus.
Ms Magnus will swap places with current station resident Greg Chamitoff, and stay on the ISS until the spring.
Also among the cargo is a water regeneration system that distils, filters, ionises and oxidises wastewater – including urine – into fresh water for drinking.
The equipment has been packed inside refrigerator-sized racks that require forklifts to lift them on Earth; but in space, a single astronaut can move a rack around with little problem.
Endeavour and its crew are to due to land back at Kennedy on 30 November.
The flight is the fourth and final mission of the year.
Nasa had hoped to fly a servicing call to the Hubble Space Telescope last month but delayed the mission to May 2009 to prepare for some additional repair work on the observatory.
In all, Nasa plans 10 more shuttle flights before the fleet is retired in 2010.