NASA abruptly called off space shuttle Endeavour's final launch Friday because of a puzzling heater failure in a critical power unit, disappointing huge crowds converging on the area for the afternoon liftoff.

President Barack Obama and his family were planning to watch Endeavour blast off. It would have been the first time in NASA history that a sitting president and his family witnessed a launch. Already at Cape Canaveral for the liftoff was wounded Rep. Gabrielle Giffords, wife of the shuttle's commander.

Launch commentator George Diller said the next try likely would be Monday at the earliest. Officials huddled in launch control, discussing a path forward.

"They are trying to assess what all is going to be required and how much time we're going to need," Diller said. "We still don't know what's wrong or why these multiple heater failures occurred."

Commander Mark Kelly and his crew were already on their way to the launch pad, when NASA announced the delay. The astronauts' van did a U-turn at the launch control center, and returned the astronauts to crew quarters.

The first family had not yet arrived in Florida when launch was canceled.

Around noon (1600 GMT), NASA reported that two heaters on an auxiliary power unit had failed. Engineers could not understand the problem and were uncomfortable about proceeding with the 3:47 p.m. (1947 GMT) launch. An electrical short was suspected.

Three power units provide hydraulic pressure to the main engines at liftoff and to the rudder and speed brake during landing. They are crucial components; each must be working perfectly before launch.

The news took guests by surprise as well as journalists who were outside watching the astrovan drive by. There was confusion when the van pulled into the driveway in front of launch control, rather than head straight for the pad three miles (five kilometers) away. Then the official announcement came over the speakers.

Endeavour's mission to the International Space Station is the last in its 19-year history. The shuttle was built to replace Challenger, destroyed during liftoff in 1986, and made its maiden voyage six years later.

Obama will still travel to Cape Canaveral Friday to tour an orbiter processing facility at NASA.

It's unclear whether Obama will meet with fellow Democrat Giffords when he is in Florida.

From Cape Canaveral, Obama will head to south Florida to deliver a commencement address at Miami Dade College.

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NASA: http://www.nasa.gov/shuttle