NASA officials have again postponed the launch of Endeavour because of potential lightning strikes, keeping Canadian astronaut Julie Payette on the ground. NASA will make another attempt on Monday.

The shuttle had been scheduled to lift off from Cape Canaveral, Fla., at 7:13 p.m. ET, but the launch has now been delayed until Monday at 6:51 p.m.

"We got the vehicle ready, and the weather unfortunately did not co-operate with us today," NASA launch director Pete Nickolenko said to the Endeavour crew, who were strapped in and ready for launch.

"We understand and we'll be ready," replied Mark Polansky, the shuttle's commander.

It's the fourth time NASA has scrubbed Endeavour's launch, further delaying a historic first for Canada.

When Payette finally blasts off, she will join fellow Canadian Robert Thirsk at the International Space Station -- the first time two Canadian astronauts will be in space at the same time.

Thirsk arrived at the space station last month for a record-breaking six-month stay.

"They'll curse once or twice, but it's one more day," said Canadian astronaut Chris Hadfield of the crew.

Hadfield was on hand for the launch.

"This is years and years and years of effort; one more day is inconsequential. It's all just the psychology and the impact that it has on their families and friends and support people."

On Saturday, NASA delayed Endeavour's launch when thunderstorms moved into the Cape Canaveral area.

Officials said they had to inspect the launch pad to ensure it did not sustain any damage from 11 nearby lightning strikes.

In a news release issued just before midnight Saturday, officials said none of the strikes hit the shuttle or its external tank and solid rocket boosters.

Lightning did strike the lightning mast and water tower, and officials feared that electrical charges from the bolts could have damaged the shuttle's electronic systems.

"We've seen nothing so far that shows anything affected any of the systems," Mike Moses, chairman of the pre-launch Mission Management Team, said in the news release.

Endeavour's mission was delayed twice in June because of a potentially dangerous hydrogen fuel leak.

With files from The Associated Press