The space shuttle Discovery lifts off from launch pad 39-A at the Kennedy Space Center in Florida, Aug.29, 2009. [Xinhua/Reuters]

The U.S. space shuttle Discovery lifted off midnight Friday from the Kennedy Space Center in Florida after two delays, on a track to the International Space Station (ISS).

NASA TV showed the shuttle blasted off at 11:59 p.m. EDT (0359 GMT, Saturday). Its twin solid rocket boosters separated just over two minutes into the flight.

The reusable boosters falls back toward the Atlantic Ocean, where they will land under parachutes and be retrieved by recovery ships. They are equipped with cameras to record the performance of Discovery's external tank and any foam loss seen during Discovery's ascent, with the glow from their engines illuminating the shuttle during the night launch.

The rocket engines boosting Discovery and its external tank towards orbit have also shut down as planned about eight and a half minutes into flight.

Discovery was originally scheduled to lift off early Tuesday. However, Tuesday's launch attempt was called off by thunderstorms and a Wednesday try was postponed by a fuel valve trouble.

The valve is inside Discovery's aft compartment. It is used for the flow of liquid hydrogen from the external fuel tank to the main engines.

NASA resolved the fuel valve problem Friday and the weather also cooperated the planned launch.

"It looks like third time really is the charm," launch director Pete Nickolenko told shuttle commander Rick Sturckow shortly before the liftoff. "We wish you and your team good luck and Godspeed."

The shuttle will reach the ISS Sunday night.