Space Launch System rocket with its Orion crew capsule perched on top at Cape Canaveral, Florida. Reuters

Rollout of the towering Space Launch System (SLS) rocket with its Orion crew capsule perched on top marks a key milestone in US plans for renewed lunar exploration after years of setbacks, and the public's first glimpse of a space vehicle more than a decade in development.

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The process of moving the 5.75-million-ton, 32-story-tall SLS-Orion spacecraft out of its Vehicle Assembly Building at NASA's Kennedy Space Center in Cape Canaveral was scheduled to begin at 5pm EDT (2100 GMT), weather permitting.

The megarocket - standing taller than the Statue of Liberty - will be slowly trundled to Launch Pad 39B on a giant crawler-transporter, a 4-mile (6.5-km) journey expected to take about 11 hours. The spectacle will be carried live on NASA Television and the space agency's website.

NASA’s next-generation moon rocket is seen in the Vehicle Assembly Building at Cape Canaveral, Florida. Reuters

Forecasts on Wednesday called for favourable conditions along Florida's Atlantic coast.

The rollout, paving the way for NASA's uncrewed Artemis I mission around the moon and back, was delayed last month by a series of technical hurdles the space agency said it has since resolved as teams readied the rocket for the launch pad.

"We are in very good shape and ready to proceed with this roll on Thursday," Charlie Blackwell-Thompson, Artemis launch director, said earlier in the week as she briefed reporters on NASA's progress.

Once secured at the pad, the SLS-Orion ship is to be prepared for a critical pre-flight test called a "wet dress rehearsal," which will begin on April 3 and take about two days to complete.

Engineers plan to fully load the SLS core fuel tanks with super-cooled liquid hydrogen and liquid oxygen propellant and conduct a simulated launch countdown — stopping seconds before the rocket's four R-25 engines would ignite — in a top-to-bottom evaluation of the entire system.

Reuters