With the imminent debut of its Orion spacecraft, NASA is on a high not felt since the space shuttle days.

Shuttle veterans, in fact, are leading the charge in Thursday morning's two-orbit, 4½-hour test flight, meant to shake out the capsule before astronauts climb aboard — eventually, perhaps, to visit Mars.

"We haven't had this feeling in a while, since the end of the shuttle program," said Mike Sarafin, the lead flight director stationed at Mission Control in Houston. "Launching an American spacecraft from American soil and beginning something new, in this case exploring deep space."

Orion is set to fly farther than any human-rated spacecraft since the Apollo moon program, aiming for a distance of 3,600 miles, more than 14 times higher than the International Space Station.

That peak altitude will provide the necessary momentum for a 20,000-mph, 4,000-degree entry over the Pacific. Those 11 short minutes to splashdown is what NASA calls the "trial by fire," arguably the most critical part of the entire test flight. The heat shield at Orion's base, at 16.5 feet across, is the largest of its kind ever built.

Navy ships were stationed near the recovery zone off the Mexican Baja coast.

"It's an exciting time," Jeff Angermeier, ground support mission manager, said from Florida's Kennedy Space Center. "You can feel the buzz."

An estimated 26,000 guests were expected to jam Kennedy for the sunrise launch, as well as 650 journalists. (Actually, the unmanned rocket will blast off from the adjoining Cape Canaveral Air Force Station.) The space center press site was packed Wednesday with out-of-town reporters not seen here since the last shuttle flight in 2011.

NASA's Orion program manager, Mark Geyer, puts the capsule's inaugural run on a par with the formative steps of Apollo and the space shuttles.

"In the sense that we are beginning a new mission, it is, I think, consistent with ... the beginning of shuttle, the beginning of Apollo," Geyer said. "It's a new mission for us, starting in the region of the moon and then beyond."

Noted NASA Administrator Charles Bolden Jr.: "For the first time in more than 40 years, this nation is going to launch a spacecraft intended to carry humans beyond low-Earth orbit. That's a big deal."

Unlike the first space shuttle flight in 1981 — helmed by two pilots — Orion will not carry astronauts before 2021.

NASA wants to test the most critical parts of the capsule on this $370 million shakedown cruise, including the heat shield, parachutes and all the sections jettisoned during ascent and entry. The capsule also will travel through the high-radiation Van Allen belts surrounding Earth; engineers want to gauge the effects on the on-board computers.

A Delta IV rocket is hoisting Orion this time around. For Orion's next flight, around 2018, the capsule will fly atop the megarocket still in development by NASA — SLS, short for Space Launch System. Only after that will humans climb aboard; NASA hopes to send an Orion crew to an asteroid corralled in lunar orbit in the 2020s and to Mars in the 2030s.

Flying this one without humans on board took some of the edge off NASA's first-flight jitters.

Still fresh in everyone's minds is October's explosion moments after liftoff of a commercial rocket carrying supplies for the International Space Station. No one was on board that flight, either, and no one on the ground was hurt.

"It's important it's unmanned because we actually structured the test to fly the riskiest pieces of the flight," Geyer told reporters. "This is the time to do it, when it's unmanned, and so we intend to stress the systems and make sure they behave as we designed them."

Lockheed Martin Corp. is handling the test flight for NASA, much the way NASA has contracted out the development of two manned capsules meant for space station ferry trips.

Even so, the shuttle ties run deep.

Sarafin's entire team at Johnson Space Center in Houston is comprised of former shuttle flight controllers. A special VIP, on hand in Mission Control on Thursday, will be Gene Kranz, the legendary flight director of Apollo 11 and 13 fame, and more.

The mission director for Thursday's flight, based at Kennedy, is Lockheed Martin's Bryan Austin, a former shuttle flight director like Sarafin.

Kennedy's director, Robert Cabana, is a former shuttle commander. So is Bolden, NASA's top guy.

"Getting the band back together," Sarafin said.