Nasa's Artemis Moon rocket rolls back to pad for possible April launch

Nasa's giant Moon rocket has returned to its launch pad for a second time, ahead of a mission to send astronauts around the Moon – something not attempted in more than 50 years.

The 98m tall Space Launch System (SLS) and Orion spacecraft made a four mile journey from their assembly building to Pad 39B at the Kennedy Space Center in Florida.

The move comes after a problem with the rocket's helium system forced Nasa to abandon a launch attempt in March and return the vehicle indoors for repairs.

Engineers say the issue has now been fixed and hope that a series of final tests at the pad will confirm the rocket is ready for an early‑April launch window.

At 00:20 local time (04:20 GMT), in darkness, the SLS emerged from the Vehicle Assembly Building (VAB) – one of the largest buildings in the world – and began its slow crawl towards the Atlantic coast.

Standing taller than Big Ben's clock tower at nearly 100m, the rocket and its launch platform weigh about 5,000 tonnes. They were carried by Crawler‑Transporter‑2, a low‑slung, tank‑like vehicle on caterpillar tracks that Nasa built in 1965 to inch Saturn V Moon rockets to the pad.

Its maximum speed is about 1mph (1.6km/h), and it crawls even more slowly on bends and up the gentle ramp to the launch pad, so the four‑mile journey can take up to 12 hours.

The snail's pace is intentional: as if carrying a precious Ming vase, the slow, gentle motion reduces stresses on the multi‑billion‑dollar rocket and launch tower. The slow speed also gives flight teams the best chance of spotting and stopping any unwelcome movement of what is in effect a mobile skyscraper.

Now at the pad, engineers will spend several days checking that the repairs carried out in the VAB have worked as intended and that nothing has shifted during the slow journey out. They will plug the launch tower back into the vehicle, run pressure tests on the helium system that caused the earlier problem.

Controllers will also rehearse parts of the countdown, sending commands through the same computers and networks that will be used on launch day, but without filling the tanks with fuel.

Once those tests are complete, Nasa's mission management team will meet a few days before the earliest launch opportunity on 1 April to review the data and decide whether to proceed.

The Artemis II crew – Reid Wiseman, Victor Glover, Christina Koch and Jeremy Hansen – have entered pre-flight quarantine and will travel to Florida closer to launch to take part in various rehearsals, including suiting up and travelling to the pad.

Nasa is working towards launch opportunities in the first week of April for the ten day mission, which will loop around the far side of the Moon before returning to Earth.

This is the second time the Artemis II rocket has made the trip to the pad. In March, Nasa had to roll it back to the assembly building after an interruption in the flow of helium to the upper stage was detected during a fuelling test.

Helium is used to pressurise propellant tanks, and any fault in that system could affect the performance of the upper stage engine or the safe draining of the fuel.

Rather than pressing ahead, managers opted to stand down from the planned launch window, return the SLS to the VAB so that engineers could have full access to the problem area rather than attempt to investigate while it was on the pad.

Inside the VAB, work platforms were raised around the upper stage so specialists could reach valves and plumbing in the helium circuit. Engineers replaced suspect components, swapped batteries in several critical systems and then repeated their tests to confirm the fault had been cleared.

After reviewing the data, Nasa managers signed off on a second rollout and the next phase of checks, which take place at the pad. If those tests are completed without incident, Artemis II will become the first crewed mission in the programme.

That will pave the way for another crewed test flight, Artemis III in Earth orbit - scheduled for 2027 - ahead of Artemis IV, which is planned for 2028 and aims to see astronauts set foot on the Moon.

For the first launch opportunity for Artemis II, Nasa is targeting 18:24 Eastern Daylight Time on 1 April, which is 23:24 in the UK. If that attempt is delayed, further launch windows currently run on 2, 3, 4, 5 and 6 April. If those are missed then the month's final opportunity in 30 April.