CAPE CANAVERAL, Fla. - Kennedy Space Center is a seaside symbol of man's scientific ingenuity, but nature flexed a set of mightier muscles yet again on Monday by preventing the launch of the space shuttle Endeavour.

A menacing evening thunderstorm, as commonplace in mid-summer Florida as surf shops and alligators, stopped Canada's Julie Payette and six other astronauts from heading to the international space station.

NASA has scrubbed another launch scheduled for Tuesday, but will try to send up Endeavour for a sixth time on Wednesday just after 6 p.m. ET.

The mission is treading close to record-breaking territory with its fifth delay.

The 127th space shuttle mission is now approaching missions 73 and 61 -- in 1995 and 1986 respectively -- for the most number of scrubs.

Both involved the ill-fated Columbia, the shuttle that disintegrated in 2003 upon re-entry into the Earth's atmosphere. The 1995 mission was postponed seven times before finally blasting off, while the '86 voyage faced six delays before lift-off.

"That's not a record you want to break," said Canadian astronaut Chris Hadfield, himself a space shuttle veteran and the first Canuck to walk in space.

"But we'll go as soon as we possibly can, when things are safe."

This month's three weather-related scrubs come despite mornings that routinely dawn bright and brilliant on Florida's Atlantic coast.

Endeavour's launch was cancelled twice in June due to a potentially hazardous hydrogen fuel leak, but this month, delays have been all Mother Nature's handiwork.

A Saturday launch was scrubbed after a violent electrical storm resulted in 11 lightning bolts touching down near the launch pad.

Sunday's launch? An approaching thunderstorm from the west. On Monday, a similar tempest arrived from the north.

Space shuttles launch as scheduled only an estimated 40 per cent of the time, and poor weather is the culprit in about a third of all delays.

Even woodpeckers -- birds that thrive in the national wildlife refuge where Kennedy Space Center is located -- once foiled a mission after boring more than 200 holes into the foam insulation of Discovery's external fuel tank.

One Canadian Space Agency official mused about why NASA officials chose Florida -- the thunderstorm capital of the United States -- instead of a remote desert area for the space centre's locale.

"Sometimes you have to wonder what they were thinking, putting it here and oceanside, instead of out in the desert in Nevada where conditions are much more steady and predictable," he said over the weekend.

But Hadfield pointed out that the space station needs to be as close to the equator as possible in order to optimize speed and fuel. The seaside location also allows the shuttle to shed its rockets over the Atlantic Ocean instead of a populated land mass.

In the event of a disaster, the shuttle will also break up over water.

Endeavour was scheduled to launch at 6:51 p.m. ET on Monday following an enticingly close attempt to blast off a day earlier. But the weather forecast had offered only a 40 per cent chance that Endeavour would fly.

Nonetheless, by the end of a partly cloudy day, the skies began to clear and Hadfield expressed cautious optimism that the launch might finally happen.

"It's not looking too bad," he said with a broad smile about an hour before the launch. "I think we might get lucky."

That was moments before a mammoth black cloud, shooting spectacular lightning bolts, suddenly appeared to the north.

Canada's newest astronauts, Jeremy Hansen and David Saint-Jacques, watched from a rooftop at the space center as the storm put on a show about a half hour before launch time.

When a reporter expressed sympathy for the Japanese astronaut who's been in space for six months and is slated to return to Earth aboard Endeavour, Hansen was surprised.

"No, he must be thrilled. He's in space, and he gets to stay," he said.

Payette has long insisted astronauts don't get discouraged by delays or bad weather, but the 45-year-old native Montrealer was more subdued as she headed to the launch pad on Monday than she had been a day earlier.

"Morale is good ... the crew is in great shape and all is well," Payette had written on the Canadian Space Agency's website after the Sunday launch was postponed.

"This is our work. We understand the delays and know we will eventually launch."

NASA's hands have been tied regarding the evening launches at Cape Canaveral. Because of the orbital mechanics required to hook up with the space station, the shuttle has only five minutes at specific times of day to blast off.

If the shuttle doesn't launch on Wednesday, it won't get another chance until late July. NASA has to make way for an unmanned Russian spacecraft.

"We'll just keep trying until we finally launch," Hadfield said. "We're not going anywhere."

Whenever Endeavour finally gets off the ground, the mission will be Payette's second voyage to the heavens. She was aboard the space shuttle Discovery in 1999.

On this mission, she's tasked with operating the shuttle's three robotic arms, two of which are Canadian-made.

The Endeavour mission will also mark a historic moment for Canada when Payette meets up at the space station with fellow Canadian astronaut Robert Thirsk.

It will be the first time Canada has had two astronauts in space at the same time.

Thirsk, of New Westminster, B.C., is spending six months at the space station, laying the groundwork for deploying Canadian robots onto other planets.

Weather conditions must be almost perfect for NASA to launch the shuttle -- rain, low cloud cover, nearby thunderstorms and high winds will result in liftoff being scrubbed.

"If you look at all of the things that have to work in order to launch, including the weather ... you would conclude it's crazy to even try," former astronaut Steve Hawley once said.

Hawley would know -- his five shuttle missions experienced 13 delays.

Technical problems also commonly keep the shuttle on the ground. A fully loaded shuttle weighs almost 590,000 kilograms and includes complex circuitry and moving parts that can malfunction.

In the past, tiny failures have resulted in disasters like the destruction of Columbia six years ago and Challenger in 1986.

The Endeavour mission will feature five spacewalks and complete construction of Japan's Kibo laboratory. Astronauts will attach a platform to the outside of the Japanese module that will allow experiments to be exposed to space.

Payette plans to take treats from home to space -- including maple cookies, maple butter and Alberta beef jerky.