Thunderstorms have again forced NASA to postpone the launch of the space shuttle Endeavour, marking the latest in a month of setbacks that have grounded Canadian astronaut Julie Payette and her six crew mates.

Endeavour was set to blast off at 6:51 p.m. ET Monday, but officials scrubbed the launch only minutes before the set time. The next possible lift-off is set for Wednesday.

If that also fails, NASA may have to hold off while an unmanned Russian spacecraft blasts off to bring supplies to the space station crew

The seven-member Endeavour crew can then try again on July 27.

The Endeavour mission will 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 arrived at the space station last month for a record-breaking six-month stay.

On Sunday, Thunderstorms forced NASA to delay the launch of space shuttle Endeavour for a fourth time when a rain cloud drifted towards its launch pad.

Space educator Randy Attwood says NASA was concerned about two things: lightning and maintaining a dry landing strip.

After the Apollo 12 rocket was hit by lightning twice during its launch through a relatively benign rainstorm, NASA realized that the rocket plume itself can generate its own electricity.

"They have a strict rule now that they can't have any kind of lightning within 30 kilometres of the launch pad," Attwood explained to Canada AM.

The other concern was the landing strip. If, for some reason, the shuttle had to turn around and land within 25 minutes of launch - which has never happened before - it can't fly through rainstorms, Attwood said.

"That would just shear off its protective tiles, which make the aeronautical shape of the shuttle. So while flying through a rainstorm in a regular airplane is no problem, for the shuttle it would be a disaster,' he said.

Marcotte says while the delays are frustrating, there is no possibility the mission will be scrubbed altogether.

"They will keep trying. It's a mission that needs to go the space station. This is the next module that needs to go up. It's the external platform to the Japanese Kibo laboratory," he explained.

When it finally goes ahead, the mission will be Payette's second voyage to space; she was aboard the space shuttle Discovery in 1999.

With files from The Associated Press