Grounded: Endeavour Space Shuttle Launch Cancelled

CAPE CANAVERAL, Fla. (AP) -- NASA has called off Friday's launch of space shuttle Endeavour because of a heater failure.

Commander Mark Kelly and his crew were on their way to the launch pad, when NASA halted the countdown. The astronauts' van did a U-turn, and returned the astronauts to crew quarters.

NASA spokesman George Dillard says the next try will be Sunday at the earliest.

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.