The seven astronauts were once again sitting inside the space shuttle waiting for the green light when liftoff was canceled two hours before its 3:26 pm (1926 GMT) scheduled time.

"We have in fact scrubbed our launch attempt for today," said NASA spokesman Bruce Buckingham.

The space agency will try again Tuesday at 2:38 pm (1838 GMT), during the July 4 national holiday marking US independence, Buckingham said. NASA officials had considered attempting another launch Monday.

Storms and concerns over lightning prompted the delay. Saturday's launch was canceled minutes before its scheduled liftoff due to similar worries.

If the weather finally cooperates, the five men and two women astronauts will fly to the International Space Station (ISS) on a mission aimed at improving shuttle safety three years after the Columbia disaster.

The ISS mission is scheduled to last about 12 days, but NASA wants to extend it by a day. Discovery has a July 1-19 launch window.

If the launch finally gets the green light, the astronauts will take critical supplies to the ISS, undertake at least two space walks, and carry out tests on new equipment and procedures to improve shuttle safety in just the second flight since the Columbia shuttle disintegrated while returning to Earth.

Columbia's demise was caused by a piece of foam that peeled off its external fuel tank during liftoff and pierced the shuttle's protective heat shield.

Debris also shed from Discovery's tank in last year's first post-disaster flight, but missed the shuttle. Still, NASA grounded the shuttle fleet until now to make further modifications to the troublesome fuel tank.

NASA Administrator Michael Griffin again defended his controversial decision to launch the spacecraft despite concerns expressed by his top safety official and chief engineer.

"I had spent an awful lot of time of my own studying the issue very carefully, because I knew it was going to be controversial, and my analysis of it convinces me that the chances of damaging the orbiter are quite small," Griffin told Fox News Sunday.

"And this is a risk worth taking at this time to get us back on track in space," he said.

NASA chief safety officer Bryan O'Connor and chief engineer Chris Scolese had called for a six-month delay to the launch to redesign foam on the fuel tank.

But the two officials backed the launch after NASA said the astronauts could take refuge on the ISS and wait for a rescue mission should the shuttle suffer irreparable damage.

The space agency has installed cameras on the shuttle and its fuel tank to detect any loss of foam during liftoff.

The shuttle's inspection will continue in space as Discovery will perform a backflip before docking to the ISS so space station crew members can photograph its heat shield.

The images will show whether NASA's fuel tank modifications have succeeded, Griffin said.

NASA's chief has said the Discovery mission was crucial to returning to regular shuttle flights. Griffin wants to conduct four shuttle missions a year to complete the space station by 2010, when the 25-year-old fleet is scheduled to retire.

Finishing the ISS is crucial to US space exploration ambitions, which include returning astronauts to the Moon and eventually sending a manned mission to Mars, he said.

"It's about creating a toehold off the surface of the Earth and using it as a stepping stone to Mars, which is a long-term goal for not only NASA and the United States, but for all mankind," he told Fox News.

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