NASA canceled Tuesday's launch of the space shuttle Discovery because of an equipment failure less than six hours before it was to blast off on a "Star Wars" mission.

NASA spokeswoman Lisa Malone said the launch would not be rescheduled before this weekend at the earliest."Right now, we've got teams of engineers in the launch control center trying to figure out where we go from here," she said.

Workers were almost finished pumping more than a half-million gallons of liquid hydrogen and liquid oxyen fuel into the space shuttle when the launch was called off at 11:15 p.m. MDT. Mission managers made the decision after learning that an engine sensor had failed.

The sensor measures pressure on a high-pressure oxydizer turbopump on one of the three main engines.

NASA quickly began draining the fuel from Discovery's big external tank. Engineers planned to enter the shuttle's rear compartment Thursday to get a closer look at the problem.

"Anytime you've got an initial problem like this, it makes you wonder what caused that and what else there might be," Malone said. "So we're going to be pretty thorough when we go back into the aft and check things out."

Discovery had been poised to blast off at 9:05 a.m. MDT on the first open military flight of a shuttle. Four of the seven astronauts were still asleep when the launch was called off.