On Saturday, NASA scheduled Discovery and its crew of seven, including a Japanese astronaut, for a blast-off from the Kennedy Space Center in Florida to the International Space Station (ISS) at 7:43 pm (2343 GMT) Sunday, with a 10-minute launch window.

"From a broad safety perspective we are fine.... We feel good about it," shuttle launch director Mike Leinbach told a press conference.

He said that equipment replacement work was about three hours behind schedule but would not delay the launch, and that leak checks would be performed later Saturday.

But he acknowledged that "no, it's not usual" when asked whether it was normal to fly without resolving a problem such as the hydrogen leak.

"We like to know the root cause for problems. I am a little surprised that we did not find something more obvious because it was a healthy leak," he said, referring to the leak NASA discovered Wednesday in the filling system for the shuttle's external fuel tank just hours before liftoff.

Mike Moses, the head of the mission management team, noted that the leak occurred on the "ground side system," not on the shuttle or the external fuel tank itself.

"I think we're in really good shape," Moses said. "It's really just a (scheduling) risk and we are comfortable proceeding on to launch to tank tomorrow, with the potential risk if we don't find the root cause, it could reoccur and at which point we scrub again."

The leak was found when the external tank was 98 percent full of liquid hydrogen. The tank had to be emptied again for the checks to be carried out, and all components in the filling system were to be replaced and a battery of precautionary checks and tests conducted.

"We did everything we could, which is to replace all the hardware and now we are going to see if it's an alignment issue or not."

NASA has until Tuesday to launch the shuttle in order not to hamper a Russian Soyuz mission to the ISS that is due to lift off on March 26, carrying American businessman Charles Simonyi. Two spacecraft cannot dock at the space station at once.

If Discovery is unable to launch by Tuesday, then its mission will be delayed until April, which will also put back other shuttle flights due in May to repair and maintain the Hubble telescope.

Meteorologists are calling for good weather Sunday for coastal Florida and an 80 percent chance of favorable launch conditions.

A Sunday launch would shorten the 14-day mission to 13 days, and force NASA to scrap one of its four planned spacewalks but the adjustments should not affect the mission to deliver and install a fourth pair of solar panels to the ISS.

The panels are to supply power for onboard laboratories and more power for the station's crew, which will double from three to six in May.

Installing the panels on the 100-billion-dollar station was to take a two-astronaut team four space walks of more than six hours each to complete, according to NASA's original plans.

The pairs of solar panels, containing 32,800 solar cells, are each 35 meters (115 feet) long. And the final array, once in place, should boost available energy to the ISS to 120 kilowatts from the current 90.

The extra power will help run the expanded array of scientific experiments to be conducted in the ISS, which saw the addition over the past year of NASA workspace and a pair of international laboratories -- Europe's Columbus and Japan's Kibo.

Discovery's astronauts include Koichi Wakata of the Japan Aerospace Exploration Agency, who is to become the first Japanese crew member on the station.

After Discovery finally docks at the ISS, Wakata is to stay aboard the station, while US astronaut Sandy Magnus -- who arrived at the ISS aboard the shuttle Endeavour in November 2008 -- will return home.

earlier related report
NASA eyes new Discovery launch try on Sunday
NASA Friday appeared likely to make a new attempt to launch the space shuttle Discovery on Sunday even if the cause of the hydrogen leak that delayed its mission this week remains unknown.

"We are prepared if we found nothing to go ahead, reassemble, reconnect and get back into a launch configuration," Mike Leinbach, the launch director, told reporters at a briefing.

Technical crews were replacing parts and running precautionary checks and other tests early Friday at the site where the leak took place Wednesday scrubbing the launch for the International Space Station (ISS) planned for that day, and everything looked fine, he said.

The leak was detected when the external fuel tank was 98 percent full of liquid hydrogen stored at an ultra-low temperature. The tank had to be emptied again for the checks to be carried out.

"Once we did get in we did a very detailed measurement ... all the measurements turned out fine. Then we went to a standard leak check ... at ambient conditions, we saw no leak, which is what we were expecting," Leinbach said.

"And then we disconnect the ordinance ... We don't have any smoking gun yet. We did see one seal that may have a slit; I am not sure if it is the cause of the issue yet. You have to give us another six, to eight hours before we can declare that the culprit or not.

"We are going to change all the components in that system as we plan to do whether we found any issue or not and get back to a launch configuration."

NASA officials will meet again on Saturday at 1:00 pm (1700 GMT) to review the repairs and decide whether to give the green light for a launch on Sunday.

If the same leak occurs again, then the launch will be scrubbed again, Leinbach said. If everything is normal, then the first leak will be seen as an unexplained abnormality, he said.

The tank should be filled in an operation starting at 10:18 am (1418 GMT) Sunday. Launch was scheduled for 19:43 pm (2343 GMT) with a 10-minute window for blastoff, Leinbach said.

NASA has until Tuesday to launch the shuttle in order not to hamper a Russian Soyuz mission to the ISS that is due to lift off on March 26, as two spacecraft cannot dock at the space station at once.

If Discovery is unable to launch by Tuesday, then its mission will be delayed until April, which will also put back other shuttle flights due in May to repair and maintain the Hubble telescope.

The Discovery and its crew of seven are due to deliver and install a fourth pair of solar panels to the International Space Station.

The panels are to supply power for onboard laboratories and more power for the station's crew, which will double from three to six in May.

Discovery's astronauts include Koichi Wakata of the Japan Aerospace Exploration Agency, who is to become the first Japanese station crew member.

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