Space shuttle Endeavour sits on launch pad 39-A at the Kennedy Space Center in Cape Canaveral, Fla. early Wednesday, June 17, 2009.

NASA scrubbed Wednesday's launch of space shuttle Endeavour because of a recurring hydrogen gas leak, and the space station construction mission is now postponed until July.

The decision came just three-and-a-half hours before the planned pre-dawn launch. NASA technicians discovered that a leak they thought they had fixed, was still emitting hydrogen above levels considered to be safe.

An earlier launch had been planned for last Saturday, but was postponed when the leak was first discovered.

NASA spent the past four days trying to fix the problems with the hydrogen gas vent line that connects to the external tank.

"The leak was way out of spec again, and so we were just not comfortable pressing on," said Mike Leinbach, assistant launch director for the mission on Wednesday.

The venting system is used to carry excess hydrogen safely away from the launch pad during liftoff. Hydrogen gas is extremely volatile and can burn in large enough quantities.

CTV's Joy Malbon, reporting from Cape Canaveral, said the feeling at NASA was one of "huge disappointment."

"I mean they gave it everything they've got, the old college try," Malbon told CTV's Canada AM.

"First it was lightning in the sky, a thunderstorm, they had the weather to contend with, and then as late as about 11 p.m. they said it's good to go, most of the storm has moved off. And then they find out there's a hydrogen leak."

A similar hydrogen leak stalled a shuttle flight earlier this year when a leak occurred where a vent line hooks up to the tank. The hookup was replaced along with a couple of seals and the seepage stopped, but engineers never did determine the exact source of the trouble.

Ivan Semeniuk, of the Dunlap Institute for Astronomy and Astro Physics, said the continued problems are frustrating for NASA.

"This is the third time in the last five launches that this problem has crept up. It also happened in March. They did resolve it at that time, but this is problematic because the fix that they applied in March that worked, hasn't worked (this time), so something unusual is going on," he told Canada AM.

Payette, a 45-year-old mother of two boys, is one of seven crew members who will now have to wait until July before the 16-day Endeavour mission can begin.

Space missions are usually quickly rescheduled, but there is a "thermal blackout" period for the next few weeks because of the space station's relative position to the sun.

A delay until at least July 11 means the rest of the year's shuttle program will have to be rescheduled.

NASA is working on a tight schedule for eight more flights, before retiring the shuttle fleet and completing construction of the ISS by the end of 2010.

Endeavour's mission

Endeavour will take the final segment of Japan's huge space station lab and an outdoor shelf for experiments to the orbiting outpost.

The astronauts will also attach some spare parts outside the space station, replace aging batteries and perform other maintenance.

Payette's trip to the space station, her second trip in space, will be historic. She is due to join fellow Canadian astronaut Bob Thirsk, who boarded the space station on May 29 along with a Russian and a Belgian astronaut. He's living there for six months.

Endeavour's crew of seven is scheduled to be aboard the space station for close to two weeks, during which five space walks are planned.

Payette is making her first trip to the space station since the Discovery mission in 1999 when she became the first Canadian to visit the outpost.

Since that trip, the space laboratory has grown to the size of two football fields.

The 45-year-old mother of two boys will be a shuttle flight engineer for the first time on the current mission, keeping busy by helping operate three robotic space arms.

One of them is the Canadarm1 on the shuttle and the other two, the Canadarm2 and a Japanese arm, are on the space station.

The Japanese robotic arm will be used to install a platform for experiments on the station's Japanese Kibo module.

Endeavour's flight comes three weeks after the successful repair effort at the Hubble Space Telescope carried out by Atlantis.

The mission may be the last time a Canadian flies on an American shuttle, before the fleet is retired.