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Systems go for Sunday Shuttle Discovery launch

After a hydrogen fuel leak grounded the first launch attempt of the spacecraft, NASA successfully met its goal of a Sunday evening liftoff.

Space Shuttle Discovery awaits its launch at NASA's Kennedy Space Center's Launch Pad 39A. NASA

Update at 4:44 PDT: Space Shuttle Discovery has successfully lifted off as scheduled.

If the weather cooperates--and forecasters predict that it will--Space Shuttle Discovery will take off for the International Space Station on Sunday evening. A hydrogen gas leak delayed a planned launch last week.

NASA said blastoff is scheduled for 7:43 p.m. EDT from Kennedy Space Center in Florida.

Commander Lee Archambault is set to lead a crew of seven on the STS-119 mission, which is aimed at installing the fourth and final set of power-generating solar arrays to the ISS. The arrays will provide electricity to fully power science experiments and support the station's expanded crew in May.

Altogether, according to NASA, the four sets of arrays can generate 84 to 120 kilowatts of electricity--enough to provide power for more than 40 average homes. Since the three existing arrays can handle the majority of the station's day-to-day operational and life support needs, the newest solar array will double the amount of power available for scientific research.

The crew of the STS-119 mission also plans to replace a failed unit for a system that converts urine to potable water.

The discovery of a hydrogen leak outside Discovery's external fuel tank halted Wednesday's launch attempt. The venting system associated with the leak is used to carry excess hydrogen safely away from the launch pad, and though NASA could find nothing broken, it has replaced all the hookups. The mission is already a month behind schedule.

NASA weather forecasters on Sunday pinned the chances of acceptable launch conditions at 80 percent.

As launch hour approaches, space buffs can follow regular updates here on NASA's preparations to get the craft into space. Last I checked, the launch team was monitoring a drop in helium pressure in a liquid hydrogen umbilical disconnect. Launch Director Mike Leinbach has sent a team of specially trained personnel to the pad to make manual adjustments to increase the pressure. NASA said the issue will not interfere with Sunday's launch attempt.

Fingers crossed.

Crew heads to Astrovan
Discovery crew members wave as they make their way to the Astrovan that will take them to Launch Pad 39A at Kennedy Space Center in Florida. After a launch delay Wednesday, NASA hopes to get the shuttle aloft Sunday evening. NASA TV