A half century after the end of NASA's Apollo era, the U.S. space agency's long-anticipated bid to return astronauts to the moon's surface remains at least three years away, with much of the necessary hardware still on the drawing board.

But NASA aims to take a giant leap in its renewed lunar ambitions with the debut launch set for next Monday in Florida of its next-generation megarocket, the Space Launch System (SLS) and the Orion crew capsule it is designed to carry.

The combined SLS-Orion spacecraft is due for blastoff from the Kennedy Space Center in Cape Canaveral, sending the uncrewed capsule around the moon and back to Earth on a six-week test flight called Artemis I.

"We are go for launch," NASA Associate Administrator Bob Cabana, a former space shuttle pilot and commander, told a news briefing late on Monday following the mission's flight readiness review.

The journey is intended to put the SLS vehicle, considered the world's most complex and powerful rocketship, through a rigorous stress test of its systems during an actual flight before it is deemed ready to carry astronauts.

The SLS represents the biggest new vertical launch system NASA has built since the Saturn V rockets flown during its Apollo moon program of the 1960s and 1970s.

More than a decade in development with years of delays and billions of dollars in cost overruns, the SLS-Orion spacecraft so far has cost NASA at least $37 billion, including design, construction, testing and ground facilities. NASA Administrator Bill Nelson has called the Artemis program an "economic engine," noting that in 2019 alone, for example, it generated $14 billion in commerce and supported 70,000 American jobs.

Congress has steadily increased NASA's budget to include funds for Artemis. Among the greatest financial beneficiaries are the principal SLS and Orion contractors - Boeing Co BA.N and Lockheed Martin Corp LMT.N, respectively.

NASA's Artemis program, named for the goddess who was Apollo's twin sister in ancient Greek mythology, aims to return astronauts to the moon as early as 2025 and establish a long-term lunar colony as a steppingstone to even-more-ambitious future voyages sending people to Mars. 

"Even with this delay and increased budget, it is doubtful that NASA will be landing humans on the moon by 2025, but if all goes well, it could happen in the next few years," Lori Garver, who served as NASA's deputy administrator during the rocket's conception, told Reuters.

LUNAR FOOTPRINTS

Twelve astronauts walked on the moon during six Apollo missions from 1969 to 1972, the only spaceflights yet to place people on the lunar surface. All of those explored regions around the lunar equator.

NASA last Friday announced 13 potential landing zones around the lunar south pole where it plans to send its new generation of explorers, including the first woman and first person of color to set foot on the moon.

A successful SLS-Orion launch is a crucial first step. The towering spacecraft, 322 feet (98 meters) tall, was slowly trundled to Launch Pad 39B last week following weeks of final preparations and ground tests.

Barring last-minute technical glitches or unfavorable weather, the four main SLS engines and its solid-rocket boosters are set to ignite at 8:33 a.m. EDT (1233 GMT) on Monday, sending the spacecraft streaking skyward. Should the countdown be delayed beyond the two-hour window targeted for liftoff, NASA has set Sept. 2 and Sept. 5 as alternative launch dates.

Following separation from the rocket's upper stage more than 2,300 miles (3,700 km) from Earth, Orion's thrusters are due to fire to set the capsule on its outbound course, bringing it as close as about 60 miles (100 km) from the lunar surface before traveling roughly 40,000 miles (64,400 km) beyond the moon and back to Earth. The capsule is due for an Oct. 10 Pacific Ocean splashdown.

Orion will be carrying a simulated crew of three - one male and two female mannequins fitted with sensors to measure radiation levels that a real-life crew would encounter.

If successful, Artemis I would pave the way to a first crewed SLS-Orion mission, an out-and-back flight around the moon designated Artemis II, as early as 2024, followed a year or more later by an Artemis III trip to the lunar surface.

Artemis III will be much more complex, integrating the SLS-Orion with spacecraft to be built and flown by entrepreneur Elon Musk's company SpaceX. Those include SpaceX's heavy-duty Starship launch and lunar-landing vehicle, still under development, and components still to be constructed including an orbital fuel depot and space tankers. Even the new moon-walking suits remain to be designed.

The plan would be for a four-person Orion crew to dock in space with a SpaceX lander to ferry two astronauts to the moon's surface for nearly a week.