NASA today postponed the final launch of the space shuttle Endeavour for at least 72 hours to investigate the failure of a key piece of technical equipment.
The space agency said the heater on one of Endeavour's three Auxiliary Power Units -- devices which power the shuttle's speed brakes, elevons and landing gear -- malfunctioned as the astronauts were getting ready to board for liftoff. The shuttle would not be able to land safely without three healthy APUs.
Technicians will now drain the external fuel tank, go into the bottom of the shuttle and diagnose the problem.
"Had we not caught this pre-launch it wouldn't have been a bad day in orbit. But having caught it gives us the opportunity to fix it," said prelaunch mission manager Mike Moses at a press conference.
"Safety always comes first," said Robert Cabana, director of the Kennedy Space Center. "We'll fly when the time is right."
The six astronauts, including Navy Capt. Mark Kelly, the commander of the mission, were dressed in their orange jumpsuits and headed to the launch pad when the announcement was made. The launch had been set for 3:47 p.m. ET.
President Obama and the first family, who had planned to attend the launch, still travelled to Cape Canaveral to visit Kennedy Space Center and get an up-close look at the shuttle Atlantis before continuing to Miami for another event. Earlier in the day the Obamas surveyed some of the tornado damage in Alabama.
The delay cames as a disappointment for thousands of spectators who flooded the Florida Space Coast, hoping to catch a glimpse of Endeavour's last launch. Kelly's wife, Rep. Gabrielle Giffords, flew from rehab in Houston to see her husband go on what will probably be his last chance to travel in space.
"Bummed about the scrub!! But important to make sure everything on shuttle is working properly," Giffords' staff said via Twitter.
Giffords was shot in the head during a shooting rampage in Tucson in January that left six people dead and 13, including Giffords, injured.
Going to Florida was described as a major step for Giffords. NASA said she would watch the launch in private. It's unclear whether Giffords will now remain in Florida.
"She was very excited to not only be here...but to also be out of the hospital," Giffords' brother-in-law astronaut, Scott Kelly, said in an exclusive interview Thursday with ABC News affiliate KTRK's Kevin Quinn. "I can't think of the exact words but it's very important to her...she is very excited about it."
The president had a ten-minute meeting with Rep. Giffords, and he and his wife talked briefly with Endeavour's crew. When asked by reporters, NASA managers said the Obamas were cleared by doctors to make sure the astronauts, who are kept in a loose quarantine before a flight, would not pick up any stray infections they might take into orbit.
The 14-day mission into space, when it happens, will be the last for the space shuttle Endeavour, and could yield new clues to the origin of the universe. One more shuttle flight, by the orbiter Atlantis, is still scheduled for June 28.
Kelly and pilot Greg Johnson, spacewalkers Mike Fincke, Drew Feustel and Greg Chamitoff, and Italian astronaut Roberto Vittori are headed for the International Space Station, where they are set to conduct four spacewalks and test a new rendezvous system for Orion, the capsule NASA planned as its next-generation spacecraft.
Endeavour, its external fuel tank and solid rocket boosters will weigh more than 4.5 million pounds at liftoff, but reach 100 miles per hour in 10 seconds, NASA says.
The shuttle will head northeast, gaining speed and altitude as it skirts the Eastern Seaboard, passes over Halifax, Nova Scotia, and heads out over the Atlantic Ocean. On the way, it will jettison its rocket boosters and external fuel tank, which will plunge into the sea.
In orbit, Endeavour will reach speeds of over 17,200 miles per hour, or 25 times faster than the speed of sound.
NASA says the mission is the most scientifically significant since the flight to repair the Hubble Space Telescope two years ago.
The shuttle is carrying a $2 billion alpha magnetic spectrometer, an instrument that will be installed on the space station. It could prove or disprove the Big Bang Theory of how the universe was formed.
"We think we are going to find something really exciting, but we just don't know what it is," said Nobel prize-winning physicist Samuel Ting, whose research led to the design of the device.
Endeavour's flight will be the 134th for a U.S. space shuttle. Shuttle Discovery ended its career earlier this year; Atlantis is expected to make its trip to space in June.
NASA recently announced the retirement homes for its shuttle fleet. Endeavour will head to the California Science Center near Los Angeles after it's decommissioned. Discovery will go to the Smithsonian National Air and Space Museum outside Washington, D.C., and Atlantis will remain on display at the Kennedy Space Center in Florida.