At Launch Readiness News Conference, the Mission Management Team announced that space Shuttle Atlantis is ready to fly on Friday. The countdown was on track despite uneasiness about a possible hailstorm later Wednesday.

"We feel very good about where we are with Atlantis," shuttle launch integration manager LeRoy Cain said following a mission management review. "We're ready to go and the team is ready to go and we're just really excited to be at this point after a long and arduous spring."

Atlantis is due to launch at 2338 GMT Friday, but as the Atlantic hurricane season brews, NASA weather officials were casting a worried glance on predictions of a hail storm, with thunder and lightning, that could interfere with the fuelling.

The storm could carry hail 1.27-centimetres in diametre and winds stronger than 50 knots, said US Air Force Lieutenant Colonel Pat Barrett, the weather officer.

"We are set to load the cyrogenics ... weather may delay the process," said one NASA official in a briefing broadcast on NASA TV.

Hail damage in February to the fuel tank caused the Atlantis mission to be delayed until June. More than 2,000 dents had to be repaired.

"We would have to be so unlucky to have more hail on this tank," said another official.

Atlantis heads for the International Space Station for a familiar mission. The crew will install a new truss segment, unfurl new solar arrays and fold up an old one – all tricky stuff that’s been done on the past two missions.

“I jokingly call those flights the test flights for us,” said Kelly Beck, lead space station flight director for STS-117.

And with two successful missions leading the way, those involved with this flight are hoping it will be the best yet.

“We’re really fortunate that we have those guys to follow,” Atlantis’ commander, Rick Sturckow, said. “Almost everything went great on those missions, and the things that didn’t go so well, we’re able to learn from.”

Along with Sturckow, the crew includes Pilot Lee Archambault and mission specialists Patrick Forrester, Steven Swanson, John "Danny" Olivas, Jim Reilly and Clay Anderson, who will launch on the shuttle but remain on the station to begin a long-duration flight. Expedition 15 Flight Engineer Suni Williams, who has been aboard the station since December 2006, will return home on Atlantis.