The Space Launch System (SLS) 'megarocket' that will one day take astronauts to the moon and Mars has passed a major assembly milestone, according to NASA.
The US space agency said the ten segments that make up the two booster rockets were vertically stacked over several weeks at the Kennedy Space Center.
When launched, the $18.6 billion SLS will be the most powerful rocket ever made and capable of taking cargo and astronauts to the moon in a single trip.
Passing this milestone brings NASA a step closer to the first uncrewed Artemis mission to the moon and back - due to launch on 312ft SLS by the end of this year.
Getting the rocket off the ground for Artemis I in 2021 is critical to meet the 2024 target of landing the first woman and next man on the moon with Artemis III. .
Engineers placed the first segment of the massive rocket on November 21, 2020, and continued the process until the final nose assembly was placed on March 2.
Before the launch later in the year the core stage needs to arrive, and prior to that the team will finish installing electrical instruments and pyrotechnics.
When the SLS core stage arrives at Kennedy, technicians will stack it on the mobile launcher between the two boosters.
When it launches the Orion capsule on its journey around the moon it will be the most powerful rocket in the world - producing 8.8 million pounds of thrust.
'Seeing the Space Launch System solid rocket boosters stacked completely on the Mobile Launcher for the first time makes me proud of the entire team,' said Bruce Tilleer, SLS booster manager at the Marshall Space Flight Center.
'This team has created the tallest, most powerful boosters ever built for flight, boosters that will help launch the Artemis I mission to the Moon.'
Artemis I will be an uncrewed test of the Orion spacecraft and SLS rocket as an integrated system ahead of crewed flights to the moon in 2023. This 2023 launch will be reminiscent of Apollo 10 and is intended to act as a crewed dress rehearsal for the 2024 mission.
On-board astronauts will separate from the propulsion stage and practice manually approaching and moving away from it as practice for future missions.
They will also perform proximity operations and docking manners which will help inform the 2024 touchdown, dubbed Artemis III.
Unlike Apollo 11, where the astronauts had less than a day on the moon, Artemis III will stay there for a week.
Under the Artemis program, NASA aims to land the first woman and the next man on the moon in 2024 and establish sustainable lunar exploration.
Getting to the Moon requires a powerful rocket ship to accelerate a spacecraft fast enough to overcome the pull of Earth's gravity and set it on a precise trajectory
SLS has four powerful engines at its base and two solid rocket boosters attached to either side - allowing it to carry larger payloads to the moon than the Saturn V rockets that took the Apollo astronauts.
At liftoff, the SLS core stage and twin solid rocket boosters fire to propel the 5.75 million pound rocket off the launch pad at Kennedy Space Center in Florida and send it into orbit, carrying an uncrewed Orion spacecraft.
To do this, in a mere eight minutes, SLS's four RS-25 engines burn 735,000 gallons of liquid propellant to create two million pounds of thrust and the twin rocket boosters burn more than two million pounds of solid propellant.
Each of the two boosters that will help lift the rocket into space are divided into five segments - those pieces are what have been assembled on the mobile launcher.
This launcher will support it through testing and transfer it to the launch pad at the Kennedy Space Center later in the year when it is ready for liftoff.
'Stacking the solid rocket boosters is a huge milestone,' NASA's senior vehicle operations manager Cliff Lanham told the BBC.
'It means the rocket is being assembled on the mobile launcher and we are in the final stages of a long journey - getting to launch Artemis 1.'
SLS is still scheduled to launch Artemis 1 later this year, but the 2024 date for the first humans on the moon since 1972 is in doubt.
This 'aggressive target date' was set by the Trump administration, and while President Joe Biden backed the return to the moon, he hasn't mentioned the date.
Professor Christopher Conselice from the University of Nottingham told MailOnline the 2024 date may well be technologically possible, but not on the current budget.
'It is extremely unlikely that that deadline will be met,' he says.
'Billions of dollars more per year are required and were originally budgeted when Artemis was proposed. It's likely the goal can be reached, but not by 2024.'