CAPE CANAVERAL, Fla. --
NASA has called off Friday's launch of space shuttle Endeavour because of a technical problem.
Commander Mark Kelly and his crew were on their way to the launch pad when NASA called off the countdown due to a heater failure.
The van carrying the astronauts turned around and returned to crew quarters.
The next try will be Sunday at the earliest, said NASA spokesman George Dillard.
NASA reported earlier that two heaters on an auxiliary power unit were not working. Engineers could not understand the problem, and the launch was halted.
Kelly is married to wounded congresswoman Gabrielle Giffords. She is already in Cape Canaveral. President Barack Obama planned to watch the launch but had not yet arrived.
The Associated Press contributed to this report.