LONGUEUIL, Que. - Canadian astronaut Julie Payette apologized Saturday to her friends and family who had travelled to Florida to see her and six American crew mates lift off on board Endeavour after NASA scrubbed the space shuttle launch due to a hydrogen leak.

"I am sorry for those of you who have travelled to Florida to see the launch and hope you will be able to wait a few days for our next launch opportunity," Payette wrote in a message posted on the Canadian Space Agency's website.

"Our crew is taking things in stride and will be ready to go when called."

NASA ordered a halt shortly after midnight, less than seven hours before Endeavour was due to blast off, after a hydrogen gas leak was discovered near the external fuel tank.

A similar problem developed last March during the first attempt to launch the shuttle Discovery.

The departure of Endeavour was put off for at least four days and the U.S. space agency is expected to announce its next move on Sunday.

Launch director Mike Leinbach said the leak, located at a vent line hookup on the fuel tank, was significant.

Hydrogen gas is extremely volatile and can burn in large enough quantities, he noted.

The countdown was halted before the seven-member crew had suited up for the flight.

Fellow astronaut Chris Hadfield, who has kept in contact with Payette, says the delay is no big deal for crew members.

"The crew has been preparing for launch for years and specifically together as a crew for a year-and-a half," he said in an interview from the Kennedy Space Center.

"So proportionately a four-day delay is not a great big deal."

Hadfield said the mother of two boys is more concerned about all the Canadians, especially family and friends, who came down to Florida and took a vacation to watch the launch.

Payette last visited the space station in 1999 when there was no one on board. She was the first Canadian astronaut to voyage to the now-giant space complex.

Benoit Marcotte, the Canadian Space Agency's director of operations, agrees Payette isn't overly concerned about the launch delay.

"It's normal in their business to have those kinds of situations," he told The Canadian Press.

Hadfield also said he's not concerned that the leak is similar to the one which occurred three months ago.

"In fact it makes it easier to fix in that we have a pretty good idea of this problem," he said in an interview from Florida. "We know how we fixed it last time."

But a scheduling conflict could mean the Endeavour crew may not be able to blast off for the space station in the coming days.

Marcotte noted that NASA is also committed to launch another probe to the moon between June 17 and 18.

"That launch is pretty critical because it has to be timed perfectly with the alignment of the orbit of the moon."

Marcotte said NASA has to decide whether the moon launch is more important than launching to the space station.

"The window of opportunity for this launch stops really on the 19th or the 20th," he added.

If Endeavour isn't flying by next Saturday, it will have to wait until July 11 for the next launch attempt because of unfavourable sun angles that would make the shuttle too hot while docked at the international space station.

During the mission, the space shuttle and its crew are supposed to deliver the final segment of Japan's huge space station lab, along with some spare parts for the giant space complex.

When Endeavour finally pulls up, there will be 13 people together in orbit for the first time, including Canadian astronaut Bob Thirsk who is on a six-month visit.

Hadfield, who acted as a back-up for Thirsk, was also in Russia when his fellow Canadian space traveller blasted off in a Soyuz spacecraft on May 27.

"I've spent the majority of my life on the road for the last year and a half," he said.