In a surprise turn of events, NASA has called off the launch for the space shuttle Endeavour due to a heating problem. The launch was set to be the last one for the space shuttle before it went into retirement.

The space agency said there is an issue with the Auxiliary Power Unit 1 heaters. As a result, it is now saying there will be a minimum three day turnaround while the engineers address the issue.

The heater had a hard failure, and despite several troubleshooting attempts, Shuttle Launch Director Mike Leinbach said it wouldn't get hot enough. Heaters are required to keep the APUs' hydrazine from freezing on orbit. NASA's engineers now believe the problem might be associated with a Load Control Assembly, which is a switchbox, located in the aft end of Endeavour, or an electrical short in the wires leading into or out of the switchbox. 

He said NASA didn't want to commit to the flight with possibility of the heaters not working, as losing it in orbit would be disastrous. The heating units power hydraulics systems on the shuttle during its return to Earth.

Leinbach said NASA would scrub, and have a turnaround of at least three days. At the earliest, the shuttle will be able to launch Monday at 2:30 p.m. Technicians will get into the aft training deck and do some troubleshooting.

Unfortunately for commander Mark Kelly and his team, the orbiter isn't ready, and we have a phrase, we won't fly until we're ready, Leinbach said.

NASA is planning a press conference for 4:00 p.m. today.

The last launch of the Endeavour is not the end of a historic ride for the well-traveled craft. It will also will feature commander Mark Kelly, husband of congresswoman Gabrielle Giffords, who was nearly shot to death earlier this year. Both Giffords and  President Barack Obama were to be in attendance as Endeavour made its final journey into space. It will be Giffords' first appearance since being shot.

The 14-day mission will see the crew of the Endeavour deliver the Alpha Magnetic Spectrometer (AMS) and spare parts including two S-band communications antennas, a high-pressure gas tank, additional spare parts for Dextre to the International Space Station. After it finishes up, NASA will conclude the space shuttle program with the Atlantis. Once its done for good, Endeavour will retire to the California Space Center, in the same state where it was once built.