CAPE CANAVERAL, Fla. — Wind gusts and sticky fuel valves conspired to keep NASA’s new Orion spacecraft on the launch pad Thursday, delaying a crucial test flight meant to revitalize human exploration.

NASA promised to try again Friday morning.

The space agency’s new countdown clock got a workout as problem after problem cropped up in the final four minutes, and the count switched back and forth.

A stray boat in the launch-danger zone kicked things off badly. Then excessive wind twice halted the countdown, followed by valve trouble on the unmanned Delta IV rocket that could not be fixed in time. Declining battery power in the rocket’s video camera system reinforced the decision to quit for the day.

Orion is how NASA hopes to one day send astronauts to Mars. This inaugural flight, while just 4½ hours, will send the unmanned capsule 3,600 miles into space.

It’s the first attempt to send a spacecraft capable of carrying humans beyond a couple hundred miles of Earth since the Apollo moon program.

The ultimate goal, in the decades ahead, is to use Orion to carry people to Mars and back.

NASA anticipated 26,000 guests for the historic send-off — the roads leading into Kennedy Space Center were packed well before dawn — and the atmosphere was reminiscent of the shuttle-flying days. “Go Orion!!” urged a hotel billboard in nearby Cocoa Beach.

A Thursday launch would have been special for another reason: NASA launch commentator Mike Curie noted that it was the 16th anniversary of the launch of the first U.S. piece of the International Space Station, by shuttle Endeavour. “That was the beginning of the space station, and today is the dawn of Orion,” he said.