The shuttle Endeavour and its six-person crew will have to wait at least three more days to begin the orbiter's final journey.

Early Friday afternoon, shuttle launch control at the Kennedy Space Center in Florida canceled a planned 3:47 p.m. lift-off after engineers discovered problems associated with units that provide power to the shuttle's hydraulic systems.

Among their many duties, the hydraulic systems drive the control surfaces on the wings and tail that are crucial to guiding the craft to a safe landing after reentry. The hydraulic systems also operate landing gear and change the positions of the main-engine nozzles during launch to steer the craft during its ascent.

"We'll fly no orbiter before its time, and she just wasn't ready to go" on Friday, said shuttle launch director Michael Leinbach shortly after the scrub.

The word came as the crew, led by mission commander Mark Kelly, was on its way to the pad in the trademark silver van. In at least 72 hours, mission managers could try for another shuttle launch.

One reason the scheduled launch has been closely watched is because Captain Kelly's wife, Rep. Gabrielle Giffords (D) of Arizona, planned to be in attendance. Representative Giffords was the apparent target of a shooting spree in January that injured her and 13 others and killed six. According to the Associated Press, it has not been decided yet whether Giffords will remain in Florida for a rescheduled shuttle launch.

President Obama and his family had also planned to attend the shuttle launch. According to the Los Angeles Times, Mr. Obama will still visit Cape Canaveral on Friday as part of several stops in the region.

The shuttle problem on Friday first appeared with heaters that warm fuel lines leading to one of the orbiter's three auxiliary power units. The heaters prevent the fuel lines from freezing while the shuttle is in space. One heater failed, while another was faltering. Engineers suspect that the late-morning problem appeared because of a malfunction in the electrical system that powers them.

The main question now is how long the launch delay will last. Technicians will need to climb into the orbiter's innards to test the electrical system in question. They are expected to make their pad call on Saturday, after the orbiter is cleared of fuel and scaffolding can be set up to gain access to the suspect electrical boxes.

Endeavour's mission to the International Space Station aims to deliver spare parts as well as a 7.5-ton particle detector that scientists will use to try to answer questions about the universe's composition and evolution.

National Aeronautics and Space Administration officials say they will have a better idea of how long Endeavour will have to stand idle on the pad after a meeting with engineers and payload managers later on Friday afternoon.