The US space agency decided to return Atlantis to its seaside launchpad after the forecast improved, showing Ernesto's winds would no longer pose a threat to Atlantis, officials said.

NASA has been eager to fly Atlantis on the first International Space Station (ISS) construction mission since the 2003 Columbia tragedy and has aimed to launch the shuttle by September 7.

Launch director Mike Leinbach expressed optimism that the shuttle could be ready to launch September 6 or 7.

"We feel good about it," Leinbach told reporters. No official date has been set, he said.

A firm date depends on how soon workers can return to the Kennedy Space Center in Cape Canaveral, Florida as the site was to close Wednesday due to the storm, Leinbach said.

Ernesto was forecast to make landfall late Tuesday or early Wednesday.

The storm "is going to be less intense than first thought and it does not exceed our (wind) requirements to be at the pad," NASA spokesman Bruce Buckingham said.

Atlantis, standing on a massive mobile platform, was crawling toward its shelter when it was stopped five hours into a trek on a tree-flanked road and ordered back to the seaside launch facility.

The National Aeronautics and Space Administration has wanted to launch Atlantis by September 7 to avoid interfering with a Russian Soyuz mission.

It would have been tricky for Atlantis to be ready to launch before September 7 if it had weathered the storm inside its hangar. It takes eight days to prepare a shuttle for launch once it is placed on its launchpad.

Facing the threat of missing the September 7 launch date, officials had begun talks with their Russian counterparts to see if Atlantis could lift off after that date. The shuttle's launch window lasts until September 13.

Before the weather forecast changed and NASA sent Atlantis back to the launchpad, shuttle program manager Wayne Hale said the chances of launching in September appeared slim.

"Based on what we know today, and there are a lot of unknowns with where Ernesto is going to go and what kind of damage might result from the storm, I would say the probability of launching before ... September 13 is low," Hale told reporters. "Not zero, but low."

The next preferred launch opportunities are in late October, officials said.

Once it finally launches, Atlantis will carry six astronauts and a new 16-tonne segment with two huge solar panels to the ISS, a critical first step in completing the half-finished orbiting laboratory.

It would be the first of 16 flights planned to finish assembling the space station by 2010, when the three-shuttle fleet is set to retire.

The Columbia explosion forced a halt in the orbiting laboratory's construction and shifted the focus on improving shuttle safety.

After two Discovery shuttle flights in the past two years focused on safety, NASA declared it was ready to resume construction of the station, which is key to the US drive to send humans to Mars.

Safety remains a priority, however, and NASA wants to launch the shuttle during daylight to take pictures of possible debris that could strike the orbiter's heat shield during liftoff.

The agency would have two to four daylight launch days in late October, officials said.

The Atlantis flight will be the third shuttle flight since the Columbia disaster, which was caused by debris that struck its heat shield during liftoff, dooming its return home with seven astronauts aboard.

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