"We'll try to launch Friday," said Dean Acosta, spokesman for the National Aeronautics and Space Administration.

The Atlantis was set for launch Wednesday but the blast off was delayed twice because one of the three fuel cells providing electricity to the shuttle had malfunctioned.

NASA was anxious to proceed because if Atlantis does not launch on Friday its 11-day mission could be delayed until late October, as it would interfere with a scheduled mission to the International Space Station (ISS) by a Russian Soyuz space craft.

Atlantis had been scheduled to blast off on August 27, but was delayed by the effects of Tropical Storm Ernesto that pounded Florida with heavy rain and winds, then by technical problems.

NASA officials said there was a 70 percent chance of clear weather at launch time.

On Wednesday, mission manager Wayne Hale told a news conference that NASA wanted to take a prudent approach in resolving the fuel cell problem.

"We need to get some satisfactory answers to some of our engineering questions because we want to fly a good mission ... so we need to have all three of our fuel cells working properly for the full mission duration," Hale said.

The mission is to be Atlantis's first ISS construction mission in nearly four years.

The agency plans to undertake 16 shuttle missions to complete the complex assembly of the half-finished space station by 2010, when the three-shuttle fleet is set to retire.

Atlantis is to take a new 16-tonne segment with two huge solar panels that will double the station's ability to produce power from sunlight and ultimately provide a quarter of the completed ISS's power.

After the Space Shuttle Discovery returned safely in July from a mission aimed at improving safety, NASA declared it was ready to resume construction of the ISS, which is central to US ambitions to fly humans to Mars.

Three lengthy spacewalks are planned to install the solar arrays, which are 73 meters (240 feet) long when unfurled.

Officials said it will be the most complex work ever undertaken at the nearly eight-year-old space station and that the next few missions will only get harder.

During their 11-day mission, the six shuttle astronauts will also use a robotic arm to scan the orbiter's heat shield for potential damage from debris falling off the external fuel tank during liftoff.

The safety check has become routine since the Space Shuttle Columbia was struck by foam that peeled off from its fuel tank during liftoff, eventually causing the shuttle to disintegrate as it returned to Earth in February 2003, killing all seven astronauts on board.

The concern over debris has prompted NASA to favor daytime launches, which allow engineers to take pictures of the liftoff to detect any foam loss. The requirement limits the available launch dates.

Related Links
Space Shuttle News at Space-Travel.Com
Space Shuttle News at Space-Travel.Com

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