CAPE CANAVERAL, Fla. – NASA called off its attempt to launch the final voyage of the space shuttle Endeavour today (April 29) because of a malfunction in one of the spacecraft's critical power units.

Two heaters on one of Endeavour's auxiliary power units, which power hydraulics systems on the shuttle during its return to Earth, failed this morning, rendering the unit useless. Though Endeavour has a total of three units, NASA needs working spares of these critical systems.

"There's not a way to do the troubleshooting we need to do and stay in a countdown configuration," NASA spokesman George Diller said. "It will be at least a 48-hour scrub turnaround."

The next chance to lift off Endeavour comes Sunday (May 1) at 2:59 p.m. EDT (1922 GMT) from Launch Pad 39A here at Kennedy Space Center. However, NASA must first troubleshoot the power unit failure before it can set a new launch date. [Photos: Shuttle Endeavour's Final Voyage]

"We will put together a troubleshooting plan," Diller said.

NASA called off today's launch at 12:19 p.m. EDT, after Endeavour was fully fueled and the astronauts were suited up in their bright orange launch and entry suits.

Among the disappointed hopeful viewers of the launch are President Obama and his family, who were planning to watch Friday's shuttle liftoff. There's no word yet on whether Obama will still attend the next launch attempt.

Many thousands of others will also have to postpone their shuttle-viewing plans. Florida's Space Coast was expecting roughly three-quarters of a million people to come see Endeavour blast off one last time on Friday.

The space shuttle Endeavour is slated to carry six astronauts, a $2 million astrophysics experiment, and a load of spare parts to the International Space Station. The 14-day flight will feature four ambitious spacewalks outside the orbiting laboratory.

Endeavour's veteran crew will be led by Mark Kelly, husband of wounded Congresswoman Gabrielle Giffords, D-Ariz. Giffords, who is recovering from a bullet wound she received during a January mass shooting outside a Tucson grocery store. Giffords is here at Kennedy Space Center to watch her husband launch into space.

This article was reprinted with permission from SPACE.com.

Related on SPACE.com: