NASA filled up Discovery's fuel tank Friday as six American astronauts and one Swede prepared for liftoff at the fourth attempt on a mission to the International Space Station.
The shuttle was scheduled to blast off at 11:59 pm (0359 GMT) from launchpad 39B at the Kennedy Space Center near Cape Canaveral, Florida with meteorologists predicting a 60 percent chance of favorable conditions.
The process of filling the shuttle's external tank with nearly two million liters (500,000 gallons) of liquid hydrogen and oxygen began at 2:45 pm (1845 GMT) after a slight delay due to stormy weather near the launch site.
Allard Beutel, a spokesman for the Kennedy Space Center told NASA television that conditions looked good and the launch crew were "not working any technical issue."
Tuesday's first attempt was cancelled shortly before liftoff when weather conditions were deemed too dangerous, and two subsequent attempts were thwarted by problems with a liquid hydrogen fill-and-drain valve.
NASA engineers eventually discovered that the problem was caused by false instrument readings and devised a method of determining if the valve was properly closed in case the instruments malfunction again.
The filling of the shuttle's fuel tank, which takes about three hours, marked the beginning of the final preparations for launch.
The tank, painted a bright orange color and attached to the outside of the shuttle, is 46.9 meters (154 feet) high -- that is roughly the same as the Statue of Liberty -- and 8.4 meters in diameter.
Once the shuttle takes off it is expected to spend 13 days in space on a mission to deliver equipment and a new team member to the International Space Station (ISS).
Astronaut Nicole Stott is to take the place of American Tim Kopra, who will ride the shuttle back to Earth.
Also aboard the Discovery is a new freezer for the ISS, which will be used to store samples of blood, urine and other materials that will eventually be taken back for studies on the effects of zero-gravity.
Exercise is important for astronauts spending long periods of time in space, because zero-gravity can result in muscle atrophy.
The mission will include three scheduled spacewalks that will be taken up in part with the installation of a new liquid ammonia coolant tank.
The shuttle commander is veteran astronaut Rick Sturckow.
Source: AFP American Edition