Kennedy Space Center, Florida (CNN) -- The final launch of the space shuttle Endeavour was delayed Friday, temporarily putting off what promised to be an emotional moment for mission's commander.

Liftoff -- originally scheduled for 3:47 p.m. ET -- was delayed by at least 72 hours due to concerns with the shuttle's heating system.

As part of NASA's examination of the problem, the shuttle's external fuel tank will be drained of its oxygen and hydrogen propellants.

"The engineering team did not understand how this problem occurred and did not feel comfortable proceeding with a launch attempt," NASA spokesman George Diller said.

Endeavour's first commander reflects on shuttle's final mission

Cmdr. Mark Kelly's wife, U.S. Rep. Gabrielle Giffords, traveled to Florida to watch the shuttle's ascent. President Barack Obama and the first family were also scheduled to be in attendance.

Three months ago, Giffords was nearly killed when a would-be assassin shot her in the head during a meeting with constituents in Tucson, Arizona. It was not immediately clear if Gifford's doctors -- who are overseeing her lengthy rehabilition -- will allow her to remain in Florida for the duration of the delay.

Kelly was tapped to lead a crew that includes pilot Gregory Johnson; mission specialists Michael Fincke, Greg Chamitoff and Andrew Feustel; and European Space Agency astronaut Roberto Vittori. Liftoff.

Endeavour has logged more than 103 million miles in space, blasting off 24 times, but its 25th flight will be its last.

As NASA's shuttle program winds down with the last launch scheduled this summer, many in the astronaut corps are wrestling with what to do next. For the foreseeable future, Russian rockets will be the only way for U.S. astronauts to get to space.

Fincke spent a total of a year in space on the international space station, getting there and back twice on Russian rockets. But Friday's delayed flight will be his first on the shuttle.

"I think all of us, with all the changes that are going on, with our country's space program and NASA, all of us professional astronauts are looking into our hearts to see what we're going to do next," Fincke said.

Four spacewalks are planned for the mission. The space walkers will retrieve experiments, install new ones, refill tanks and lubricate parts at the International Space Station.

As shuttle program ends, astronauts want to keep flying

In its cargo bay, Endeavour will carry the Alpha Magnetic Spectrometer. At $1.5 billion, it is the most expensive piece of equipment a space shuttle has ever carried.

The spectrometer is designed to capture space particles like anti-matter and dark matter that scientists know very little about but believe exist in the universe. The spectrometer will be mounted outside the International Space Station. If it's successful, it could lead to a better understanding of how our universe began and evolved.

U.S. astronauts not sure where they go next

A few months ago, before his wife was shot, Kelly talked about his upcoming mission.

"Flying in space is a very difficult thing to give up," Kelly said. "I remember after my last flight thinking 'Well, maybe this is the last time I'm gonna do this.' And, you know, you go a couple of months out and you're like, 'Oh, I really hope this is not the end of my flying career.'"

Kelly said that when STS 134, the Endeavour's last flight, is over, "I'll be thinking the same thing, I can't really give this up. I've got to figure out a way to get back into space."

CNN's John Zarrella and Eric Fiegel contributed to this report.