CAPE CANAVERAL: An engine problem forced NASA on Monday to postpone for at least four days the launch of its colossal next-generation rocketship on a long-awaited debut test flight around the moon and back 50 years after Apollo's last lunar mission.

The countdown was halted about 40 minutes before launch time as the 32-story-tall, two-stage Space Launch System (SLS) rocket and its Orion crew capsule awaited liftoff from the Kennedy Space Center in Cape Canaveral, Florida.