The crew of space shuttle Endeavour's mission STS-134 departs crew quarters for launch pad 39A at the Kennedy Space Center in Cape Canaveral, Florida, April 29, 2011. [Photo/Agencies]

As many as 700,000 people were expected to crowd nearby coastal communities. For days, police have been warning of massive traffic delays. After Endeavour, there's only one more space shuttle flight - by Atlantis - before NASA ends the 30-year-old program and retires the fleet to museums.

Astronaut James Kelly, a two-time shuttle pilot, took the news in stride at the press site at Kennedy Space Center. "When you have technical problems, you delay. That's it. You delay," he said.

"Of course, it's always disappointing, especially for the crew," added astronaut Clayton Anderson, a former space station resident. "NASA has a great safety record and they're going to do it the right way. They're going to pull it back and do what's right. They have to."

Since arriving aboard a NASA jet on Wednesday, Giffords whereabouts have been keep secret. Her doctors had said she was "medically able" to travel and that they viewed the trip as part of her rehabilitation.

After the launch was called off, a close family friend said her family and staff were deciding whether Giffords should stay in Florida or return to Houston as soon as possible. The friend, who asked not to be named because he was not authorized to speak for the family, says the decision is not based on her health, but rather logistics.

In late morning, one of the prime heaters for the fuel line powering one of Endeavour's three auxiliary power units failed. At the same time, another heater was acting up.

Leinbach said both heaters need to be operating for redundancy. The power units provide hydraulic pressure to the main engines at liftoff and to the rudder and speed brake during landing.

The short appears to be in a switchbox or an electrical line leading to it, Leinbach said.

Endeavour's upcoming mission to the International Space Station is the last in its 19-year history. The shuttle was built to replace Challenger, destroyed during liftoff in 1986, and made its maiden voyage six years later.