Discovery is set to launch at midnight on Friday after problems with a fuel system valve that is forced to previous delays have been resolved, officials at NASA.

Discovery is cleared at 11:59 (0359 GMT Saturday) from Cape Canaveral, Florida, bound for the International Space Station (ISS).

"Weather remains the primary concern," says NASA.

The weather forced a delay of a planned launch on Tuesday and then the valve has problems with deleted a planned launch on Wednesday. NASA Discovery then tried to start early Friday, but decided to postpone re-examine the data relative to the valve.

NASA said it needed more time to determine whether the problem lies outside of the tank valve or an indicator showing that the valve opens when should be closed, but said Friday afternoon that the indicator showed the valve is closed and began feeding.

NASA said it needed more time to determine if the problem lies outside the fuel tank valve or an indicator showing that the valve opens when it should be closed, but said Friday afternoon meters showed that the valve closed and nutrition was started.

At the end of next year, NASA hopes to have completed the expansion of the International Space Station can be lifted to the space shuttle. Instead, it is a new spacecraft are built, which will be available in 2015 at the earliest.