The launch of a small rocket loaded with 500 pounds of scientific experiments was canceled Monday after NASA counted down to zero and nothing happened.

The cause of the problem was not immediately known.The 50-foot Prospector rocket was scheduled to blast off at 7 a.m. from Cape Canaveral Air Force Station, but computer and mechanical problems delayed liftoff nearly two hours.

The trouble appeared to be resolved and NASA launch commentator Lisa Malone began counting down for an 8:55 a.m. liftoff.

" . . . five, four, three, two, one. Zero," she said, then paused as the rocket remained on the pad. "We are standing by. We do not have motion of the vehicle."