NASA officials scrubbed the planned launch of the space shuttle Endeavour a little after noon on Friday for at least 72 hours after crews identified a problem with one of the orbiter’s auxiliary power units.

The mission’s six astronauts, including commander Mark E. Kelly, husband of Rep. Gabrielle Giffords of Arizona, were on their way to the launch pad when the delay was announced.

NASA Shuttle Launch Director Michael Leinbach said a heater used to keep the hydraulic system warm during launch was not working. He said attempts to fix the problem by the NASA ground crew and by the astronauts in the cockpit failed.

“We will not fly until we’re ready,” Leinbach said. “It’s the nature of our business.”

Three auxiliary units power Endeavor’s wing flaps and help rotate and direct the orbiter’s main engines. The problem will push the launch back until Monday afternoon at the earliest.

The decision scuttles what was expected to be an emotional moment: the first public appearance by Giffords since she was shot in the head during a shooting that killed six people and wounded 13 in Tucson in January. President Obama, the first lady and their daughters were also expected to be at Kennedy Space Center for the launch, and they toured the facility later in the afternoon despite the postponement.

Obama also spent about 20 minutes talking to the six astronauts on the crew of the Endeavor, including Kelly.

Giffords flew to Florida on Wednesday. The grainy images of her climbing the plane’s stairs with only minimal help have been a source of collective wonder.

Since the attack, Giffords has not spoken publicly, but she and her staff have frequently sent out messages through Twitter and Facebook. On Friday morning, her Twitter feed read: “Msg from Mark: It’s 8:15 am. We get on the clock at 10:30 am for a 3:47 pm launch. Weather looking pretty good.”

The weather, always a worry for shuttle launches, was a matter of concern throughout the morning. Thick black clouds brought raindrops to Cape Canaveral as final preparations were being made, but forecasters were predicting the skies would clear before launch time. Late in the morning, NASA officials had estimated that there was a 70 percent chance the launch would go off as planned.

A huge external tank was filled Friday morning with 535,000 gallons of the liquid hydrogen and liquid oxygen that was to fuel the shuttle’s ascent. During preparations for the launch, NASA detected a pressure problem in the fuel tanks for the shuttle orbital maneuvering system, which provides propulsion while the craft is in orbit. The tanks were rebalanced.

During its planned two-week mission to the international space station, Endeavour is scheduled to carry a seven-ton gizmo, known as an alpha magnetic spectrometer. The $2 billion device is a hunter of sorts; its quarry includes data on dark matter, antimatter and “strange matter.”

A 2007 analysis of NASA records by the Associated Press found that just 40 percent of space shuttle missions launch on time. Technical glitches and severe weather account for most delays. Woodpeckers triggered the oddest delay in 1995, when the birds drilled 200 holes in the insulation of an external fuel tank.

In 1986, the space shuttle Challenger exploded just after liftoff. Investigations pinpointed a leaky rubber seal in the shuttle’s rocket booster as the cause. In 2003, a piece of foam that shook loose during lift-off damaged the wing of the shuttle Columbia. The damage proved fatal when the orbiter blew apart during reentry.

Fourteen astronauts died in the two accidents.

Staff writer Perry Bacon Jr. contributed to this report.