The blastoff will mark the second-to-last mission to the International Space Station by a US shuttle before the storied program formally ends later this year with the mission of Atlantis, the final space shuttle to retire.

The US space agency confirmed Endeavour's launch time of May 16 at 8:56 am (1256 GMT), set last week after the initial launch attempt was scrubbed hours before liftoff on April 29.

"We've got extremely high confidence... there should be no threats to the 16th launch date," said NASA shuttle program launch integration manager Mike Moses.

However, the delay has pushed back the final launch of the 30-year-old space shuttle program to an undetermined date.

"We are not going to be able to tell you a launch date for 135," Moses said, referring to STS-135, the mission of the space shuttle Atlantis, which was set for June 28.

"I'm really sorry but we cannot estimate that," Moses continued, adding that NASA was considering mid-July but could offer no further confirmation until schedules could be arranged as NASA grapples with the layoffs of thousands of workers.

"We are being affected by our work force reductions," he said.

He said rumors that NASA was considering a July 4 launch were unfounded, and that NASA was never targeting the US national holiday of Independence Day as a potential liftoff day.

The problem that delayed Endeavour was traced to a short in the heater circuit associated with Endeavour's hydraulic system, NASA said.

"Technicians determined the most likely failure was inside a switchbox in the shuttle's aft compartment and associated electrical wiring connecting the switchbox to the heaters," NASA said.

The heaters keep fuel from freezing while in orbit.

NASA also extended the Endeavour mission by two days, making it a 16-day journey.

The crew of six astronauts, including Italian Roberto Vittori of the European Space Agency, are set to arrive at Kennedy Space Center in Florida on Thursday, NASA said.

Shuttle commander Mark Kelly's wife, Arizona lawmaker Gabrielle Giffords, is also expected to return to Florida to watch the liftoff.

She was granted leave by her doctors in Houston, where she is undergoing rehabilitation after being shot in the head in January, to watch the April attempt.

Endeavour will carry a $2-billion, seven-ton particle physics detector, the Alpha Magnetic Spectrometer-2, which will be left at the space station to scour the universe for dark matter and antimatter.

The US space shuttle program is set to formally end after the Atlantis mission, leaving Russia's space capsules as the sole method of transit for astronauts to and from the International Space Station.

As many as 750,000 people descended on Florida to witness the Endeavour launch on a sunny Friday in late April, and shuttle launch director Mike Leinbach said it is possible that turnout could be smaller this time.

"Some people won't come back. Monday morning at 9 am is not as attractive as Friday afternoon," Leinbach said.

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