Space shuttle Endeavour sits idle on its launch pad on Wednesday at the Kennedy Space Center in Cape Canaveral, Fla. (Terry Renna/Associated Press)

A hydrogen gas leak again forced NASA to delay Wednesday's launch of the Endeavour, which is scheduled to carry Canadian astronaut Julie Payette to the International Space Station.

It is the second time in less than a week that NASA has spotted a potentially dangerous hydrogen gas leak and delayed the launch of the space shuttle from Cape Canaveral, Fla.

Endeavour now won't blast off until July, at the earliest.

Launch officials said the leak appeared during fuelling in the hydrogen gas vent line that hooks up to the external fuel tank. It is the same location, which is in ground equipment not the shuttle, as the leak that was discovered on Saturday and caused the previous delay.

Attempts were made to fix the leak through remote commands but after more than an hour of efforts — the launch, which was scheduled for 5:40 a.m. ET, was cancelled.

"We're going to step back and figure out what the problem is and go fix it," said deputy space shuttle program manager LeRoy Cain. "And then we'll fly as soon as we're ready to safely go do that."

Potentially dangerous

Leaking hydrogen gas can be dangerous because of its flammability.

Following Saturday's delay, the hookup and two seals were replaced.

"I sure wish we could have rewarded them and the astronauts and everybody else with a launch this morning," said Mike Leinbach, who was serving as assistant launch director for this mission. "But the leak was way out of spec again, and so we were just not comfortable pressing on."

A similar small leak delayed the launch of space shuttle Discovery in March. Though that leak was fixed, officials have not yet been able to determine the cause.

Something has changed in the techniques or materials for the parts, and an engineering investigation hopefully will find out once and for all, Cain said.

The two launch scrubs cost NASA at least $1 million, primarily in fuel costs.

The 16-day mission is scheduled to deliver the third and last segment of the Japanese-designed Kibo laboratory along with hundreds of pounds of food to the space station.

When Endeavour finally flies, it will be one of the longer visits to orbiting outpost and include five spacewalks.

Flight engineer

As a flight engineer, Payette will play an important role on the mission, operating three robotic arms on the station.

Once the shuttle pulls up at the space station, there will be 13 people together in space for the first time ever.

Payette, the first Canadian to visit the space station in 1999, will join Canada's Robert Thirsk who is already aboard participating in a long-duration space flight.

It will also be the first time two Canadians will be in space at once.

Payette and the six other astronauts scheduled to go up on the shuttle were still in crew quarters when the leak was detected.

"I'm sure you all know that we postponed again," commander Mark Polansky wrote in a Twitter update. "It's a reminder that spaceflight is NOT routine. We will fly home to Houston this morning."

The astronauts will be released from quarantine and will be training on simulators to maintain their skills until the launch, said Mathieu Caron, a mission control supervisor at the Canadian Space Agency in Longueuil, Que.

"It's a very complex mission and they have to maintain the time that's available to them to rehearse and train to be ready," Caron said.

'That's what exploration is about'

Canadian astronaut Chris Hadfield said he's had the opportunity to speak with Payette and though she is a little stressed and disappointed that the launch got cancelled, she feels that it was the right decision given the leak.

"They were, if anything, kind of relieved," he said. "You have a problem, you do the right thing — you deal with it, try to learn from it and move on. That's what exploration is about and the crew feels fine about it."

NASA bumped an unmanned moon shot to give Endeavour a second chance to fly before a thermal blackout period kicks in with unfavourable sun angles preventing a launch before July 11.

The Atlas V rocket launch, featuring two science probes, is now scheduled for Friday.

Cain said it was too soon to know whether NASA will be able to make a July 11 launch.

With files from The Associated Press