WASHINGTON — The American space agency has rolled out its new giant Moon rocket for the first time.

The vehicle, known as the Space Launch System (SLS), was taken to the pad at the Kennedy Space Center in Florida to conduct a dummy countdown.

If that goes well, the rocket will be declared ready for a mission in which it will send an uncrewed test capsule around the Moon.

This could happen in the next couple of months.

Ultimately, it's hoped astronauts would climb aboard later SLS rockets to return to the Moon's surface sometime in the second half of this decade.

SLS is a colossus. A touch under 100m in height, it was designed to be more powerful than the Apollo Saturn vehicles of the late 1960s and early 1970s.

It will have the thrust to not only send astronauts far beyond Earth but additionally so much equipment and cargo that those crews could stay away for extended periods.

Thursday's rollout from Kennedy's Vehicle Assembly Building (VAB) is the rocket's debut in the sense that it's the very first time everyone has got to see all its different elements fully stacked together.

"The first rolling out of the VAB - that's really an iconic moment for this vehicle," said Tom Whitmeyer, Nasa's associate administrator for exploration systems development. "To be here for a new generation of a super-heavy-lift, exploration-class vehicle is a day to remember."

The SLS came out of the VAB just before 18:00 local Florida time.

It was attached to a support gantry known as the Mobile Launcher. This structure, which is itself 120m high and weighs 5,000 tonnes, was sitting atop the same mammoth tractor that used to move the Saturn Vs back in the day, and later the space shuttles.

The Crawler Transporter goes very slowly, with a cruising speed of just over 1km/h (under 1mph), so it was some 11 hours before the rocket arrived at the pad, a distance of 5km from the VAB. Engineers stopped and started the tractor along the way to check everything was as it should be.

SLS will now be prepared for a "wet dress rehearsal", likely to occur on 3 April.

This will see the rocket loaded with propellants and taken through a practice countdown all the way to a mere 9.4 seconds before the moment of lift-off. The "scrub" point is just before they would normally light the four big shuttle-era engines under the rocket.

Assuming everything proceeds to the satisfaction of the engineers, Nasa will then be able to set a flight date.

The end of May remains a possibility, but more likely it will be June or July.

This mission, under what Nasa terms its Artemis program, will propel the rocket's Orion crew capsule on a 26-day journey that includes an expanded orbit around the Moon. There will be no-one in the capsule for the test flight. This should happen on a second mission in a couple of years' time.

While Nasa is developing the SLS, the American rocket entrepreneur Elon Musk is preparing an even larger vehicle at his R&D facility in Texas.

He calls his giant rocket the Starship. Like SLS it has yet to have a maiden flight. Unlike SLS, Starship has been designed to be totally reusable and ought therefore to be considerably cheaper to operate.

A recent assessment from the Office of Inspector General, which audits Nasa programs, found that the first four SLS missions would each cost more than $4bn to execute - a sum of money that was described as unsustainable. — BBC