CAPE CANAVERAL, Fla. — A last-minute technical issue forced NASA officials to scrub the launch of space shuttle Endeavour at about noon on Friday, a move certain to disappoint President Barack Obama, U.S. Rep. Gabrielle Giffords and hundreds of thousands of spectators who flooded to the Space Coast.

The next launch opportunity will be 2:34 p.m. EDT on Monday.

The issue involves two of three heating units aboard Endeavour designed to keep fuel lines from freezing in space. While these fuel lines are not particularly critical for launch, they are vital in ensuring that the airplane-like machinery of Endeavour work on re-entry — such as landing gear and its rudder speed brake.

“”You don’t want frozen fuel lines, obviously,”” said NASA spokesman Allard Beutel.

A short circuit — either in a switch box controlling the heaters or one of the electrical lines in or out of the box — has emerged as a primary suspect, but NASA engineers are not 100 percent certain. Hints of a problem started appearing earlier Friday morning. It may take until Saturday morning, however, until technicians are able to access the broken piece.

“”We’re an absolute minimum three days for turnaround right now,”” said Launch Director Mike Leinbach, who made the call to scrub the launch at about 12:15 p.m., 3 1/2 hours before launch time. “”We fly no orbiter before its time and today she just wasn’t ready to go.””

A van carrying astronauts to the launch pad was turned around en route when Leinbach decided to scrub the launch.

Obama and his family arrived shortly after the mission was scrubbed and decided to go on with the Kennedy Space Center tour that had been arranged for them.

At the orbital processing facility – NASA’s tune-up shop and parking garage for the shuttle fleet – Obama chatted with Terry White, an iconic technician with a white handlebar mustache who has been working shuttles since 1979.

“”That’s a good mustache, too; I like that,”” Obama told White.

On a dry board inside the facility, someone had written in brown marker: “”Bolden — he gave us the beginning of the end,”” a reference to NASA Chief Charlie Bolden, who also was at the space center Friday.

An estimated 7,000 shuttle workers in Florida are set to lose their jobs after the last mission and anger at the administration is riding high. A NASA official erased the message soon after it was noticed.

The president talked with Giffords for about 10 minutes before he met with the full crew. Before going in to see her, the president greeted mission commander Mark Kelly — Giffords’ husband — in a corridor.

“”I bet you were hoping to see a rocket launch today,”” Kelly said. Replied Obama, “”We were hoping to see you.”” They shook hands and embraced before Kelly updated Obama on Giffords’ condition.

Meanwhile, mission management team chairman Mike Moses and the engineering team were meeting to make a clearer assessment of what can be done and how long it would take.

Leinbach said the first heater was found to not be working and engineers tried a variety of tests to try to get it working again, but failed.

“”Our trouble shooting proved it was a hard failure. We were not able to get it to come to life,”” he said. “”There was another heater upstream from that was also exhibiting some erratic behavior.””

While an orbiter still can land with one broken unit, launch requirements force NASA to fly with all three working.

Officials had predicted up to 750,000 onlookers for the launch.

Marc Lockhart of Richmond, Va., and his daughter, Kali, 10, were planning to stay in Florida for a couple of days to see if the shuttle would launch soon.

“”We’re so disappointed,”” said Lockhart, who drove 12 hours to reach the Space Coast.

“”We wanted to be — to see — a part of history.””

Chris King, 52, of Columbus, Ohio, made his first trip to see a launch two decades ago. The main engines fired up, and he remembers seeing a puff of smoke.

Then the launch was scrubbed.

This time, he’s taking it in stride.

“”I can’t get too upset about it,”” said King, an architect who is working on a sports stadium in Iraq. “”I knew it was a risk when I made the flight.””