NASA delayed Friday's scheduled launch of the space shuttle Endeavour at least until Monday because heaters on one of the ship's power units failed about four hours before liftoff.
President Barack Obama and his family had been scheduled to attend the launch and still planned to tour the Kennedy Space Center on Friday, the White House said.
The launch had been expected to draw up to 750,000 spectators to central Florida's Atlantic coast.
Among them was recuperating U.S. Representative Gabrielle Giffords, who is married to Endeavour commander Mark Kelly. Giffords was gravely injured during an assassination attempt on Jan. 8 and has not been seen publicly since then.
Endeavour had been filled with 500,000 gallons of supercold liquid oxygen and liquid hydrogen in preparation for a launch attempt at 3:47 p.m. ET. The astronauts were on the way to the launch pad when managers decided to postpone the flight.
"Unfortunate for the (Endeavour) team and Mark Kelly and his crew, but today the orbiter is not ready to fly and as we always say in this business, we will not fly before we're ready," said launch director Mike Leinbach.
Technicians began draining the shuttle's tank of the volatile fuel so engineers could begin trouble-shooting. The failed heater is on one of the shuttle's three onboard generators that provide hydraulic power. Initial attempts to revive the unit failed.
"We were not able to get it to come to life no matter what we did," Leinbach said.
He said it would be at least 72 hours before there could be another launch attempt.
Endeavour will be delivering a high-profile physics experiment to the International Space Station.
The mission is the 134th and next to last for the U.S. space shuttle program, which is ending after a cargo run by the shuttle Atlantis this summer.
Endeavour, the replacement ship for Challenger (lost in a fatal 1986 launch accident), is the youngest of NASA's three surviving spaceships and the second to be retired.
Discovery, the fleet leader, returned from its final space mission in March, and Atlantis is due to end the 30-year-old program with a launch on June 28. The shuttle Columbia was destroyed in 2003 as it prepared for landing.
Endeavour's last flight is devoted to outfitting the space station, a $100-billion project of 16 nations, with its highest profile science instrument, a $2-billion particle detector called the Alpha Magnetic Spectrometer.
Endeavour also carries a pallet of spare parts for the station, which will soon be dependent on smaller cargo ships from Russia, Europe and Japan for resupply missions. NASA has hired two U.S. companies, Space Exploration Technologies and Orbital Sciences Corp, to join the cargo runs beginning next year.