Endeavour was fuelled up, the astronauts were prepped and hundreds of thousands of people had gathered to watch the shuttle launch Friday in Cape Canaveral, Fla. But the historic liftoff was called off due to a technical issue.

The culprit, a faulty heater part, forced the agency to delay until at least Monday Endeavour's launch, which is to be its last before the U.S. shuttle program is retired.

There will be one more flight by the shuttle Atlantis before the program ends this summer. After that, U.S. astronauts will rely on Russian Soyuz rockets to travel to the ISS.

Hundreds of thousands of visitors were expected to travel to coastal viewpoints Friday in order to witness the historic moment.

Adding special significance to the launch is the fact that Navy Capt. Mark Kelly will command the mission. Kelly is married to Arizona congresswoman Gabrielle Giffords, who was shot in the head in a January assassination attempt.

Giffords had travelled to Cape Canaveral, Fla., from hospital in Houston to witness the launch. Her staff wrote on Twitter that they were "bummed out" about the delay, but added it was important to ensure the shuttle was working properly.

U.S. President Barack Obama and his family had also travelled to the launch site and would have become the first U.S. president to witness a shuttle launch, had everything gone as planned.

Instead Obama met Kelly after the launch was delayed, and visited Giffords for about 10 minutes.

Obama himself put an end to the shuttle program, instead directing NASA to focus on asteroids and Mars -- goals that are likely years from being realized.

As many as 700,000 people were expected to take in the launch near the liftoff site, which would have made it one of the largest crowds for a NASA event in the agency's history.

Ontario Lt.-Gov. David Onley was also in Cape Canaveral for the launch, after being invited to attend.

Engineers said it wasn't clear which part of the shuttle's heating system needs to be replaced. Depending on whether the problem is easy to fix, the shuttle could launch as early as Monday. Or it could be delayed past May 8, according to launch director Mike Leinbach.

There were some weather concerns Friday, mostly due to low clouds and strong crosswinds, but forecasters had called for a 70 per cent chance the flight would go ahead.

Key facts about Endeavour and its current mission:

  • Endeavour will carry a US$2 billion particle physics detector that will seek out antimatter and dark energy.
  • The six-member crew is an all-male team.
  • As many as 750,000 people are expected to watch the launch from nearby coastal communities.
  • Once Endeavour has flown its final mission, it will be at least three years before American astronauts once again head to space.
  • Endeavour was constructed beginning in 1987 as a replacement for Challenger.
  • Endeavour's first flight was in 1992 and is named after British explorer James Cook's first ship.