The shuttle Endeavour this week will make a third try at launching for a rendezvous with the International Space Station, after potentially hazardous hydrogen gas leaks twice delayed the mission, space officials said.
"I am happy to report that we are ready to proceed with launch countdown. We are ready to tank this vehicle tomorrow morning and proceed with our launch on Saturday evening," said Nasa test director Charlie Blackwell-Thompson at a news conference.
Leaks discovered on June 13 and 17 prompted Nasa to scuttle previous launch attempts, but tests following repairs to an external fuel tank last week proved problem-free.
"The test was very successful. We did not have any leaks, any issue or any anomaly," remarked Mr Blackwell-Thompson.
The flawless test means that officials can proceed with countdown for Endeavour and its seven-member crew to embark on its final mission to the ISS.
"Endeavour is in great shape. Our launch countdown preparations are going well, our work is on schedule," he continued. "We are ready to fly this mission."
Endeavour is set for a launch tomorrow at 2339 GMT from the Kennedy Space Centre at Cape Canaveral.
Nasa said a misaligned plate linking a hydrogen gas vent line with the external fuel tank had caused last month's leaks.
But officials said the major uncertainty now is whether the weather will hold up.
"We expect that we will see some afternoon thunderstorm in the area around launch time," said shuttle weather officer Kathy Winters.
"We are forecasting 60 per cent (chance of) weather prohibiting launch - which is of course, 40 per cent chances for 'go' weather."
Weather permitting, Endeavour is set to travel on a 16-day voyage to install a platform on the ISS, allowing astronauts to conduct experiments in the vacuum of space, 350 kilometres above Earth's surface.
It is the last of three missions to assemble the Japanese Kibo laboratory aboard the orbiting space station.
The race is on to finish construction before Nasa ends its shuttle missions in September next year.