NASA today postponed the final launch of the space shuttle Endeavour for the second time, putting it off until at least the end of the week to replace a switch box in Endeavour's engine compartment.
The six astronauts, including Navy Capt. Mark Kelly, the commander of the mission, traveled back to Houston from Florida.
"Things happen fast. We are now all aboard [a plane] for return to Houston. Be back in a few days. More to follow," Endeavour pilot Gregory Johnson said today on his Twitter account. Johnson and Kelly were to be joined on the Endeavour with spacewalkers Mike Fincke, Drew Feustel and Greg Chamitoff, and Italian astronaut Roberto Vittori.
On Friday the space agency pushed back Endeavour's launch because the heater on one of the shuttle's three Auxiliary Power Units -- devices that power the shuttle's speed brakes, elevons and landing gear -- malfunctioned as the astronauts were getting ready to board for liftoff.
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.
The second delay is 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 on Friday.
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.
"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 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. NASA says the Endeavour mission is the most scientifically significant since the flight to repair the Hubble Space Telescope two years ago.
The shuttle will carry 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.
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.