Nasa’s pioneering moon rocket sprang a hazardous fuel leak Saturday, throwing into doubt chances of a successful launch on a test flight that must go well before astronauts climb aboard. The Artemis 1 was poised to make a second attempt to fly on Saturday afternoon after the US space agency declared it had identified and fixed an engine issue that caused the postponement of the original launch attempt five days earlier. But as the sun rose, an over-pressure alarm sounded and the tanking operation was briefly halted. No damage occurred and the effort resumed, Nasa’s Launch Control reported. But minutes later, hydrogen fuel began leaking from the engine section at the bottom of the rocket. Nasa halted the operation, while engineers scrambled to plug what was believed to be a gap around a seal. Officials said three separate attempts to plug the leak, including warming fuel supply lines to try to create a plug, were unsuccessful. With barely three hours until the launch window opens, efforts to troubleshoot the issue were running into time pressure, and fuel system managers recommended a scrub. By late morning, only 11% of liquid hydrogen had been loaded, while 100% of liquid oxygen had already been tanked. Earlier this week, mission managers at the Kennedy Space Center in Florida had called off Monday’s liftoff with 40 minutes left on the countdown clock when a sensor indicated one of four RS-25 engines on the core stage of the mega Space Launch System (SLS) rocket was not cooling properly. A review found the problem was a faulty sensor, not a failure of the cooling system or engine itself, and the launch team has said it will be ignored if it malfunctions again during fuelling for Saturday’s planned attempt at 2.17pm EDT (7.17pm BST). “We have convinced ourselves without a shadow of a doubt that we have good-quality liquid hydrogen going through the engines,” John Honeycutt, Artemis programme manager, told a pre-launch press briefing. The engines must match the -250C (-420F) temperature of the liquid hydrogen fuel at liftoff, otherwise they could be damaged and shut down during the eight-minute ascent to low Earth orbit, he said. Nasa has set a two-hour launch window for the maiden flight of its first human-capable moon flight for 50 years, the test mission of Artemis 1 comprising a next-generation, six-person Orion capsule atop SLS, the most powerful rocket ever to leave Earth. This mission is uncrewed. But a successful 38-day flight to 40,000 miles (64,000km) beyond the moon and back, ending with a Pacific Ocean splashdown on 11 October, will pave the way for astronauts to be on board an Artemis II fly-by in 2024, then the long-anticipated next human landing, Artemis III, scheduled for 2025. Only 12 people, all American men, have ever walked on the moon, most recently on Apollo 17 in December 1972. Nasa has promised that the Artemis programme, named for Apollo’s twin sister in Greek mythology, will include lunar footprints from the first woman and first person of colour. Weather, which also would have thwarted the first launch attempt irrespective of the engine sensor issue, appears slightly more favourable for Saturday. A 60% chance of acceptable conditions at the opening of the launch window rises to 80% by its close, according to Melody Lovin, weather officer for the Space Force 45th wing. “We could have some showers approaching the coastline and maybe a crack of thunder and lightning with that,” she said. “This is certainly a threat again, a similar threat to what we had the other day. [But] I do not expect weather to be a showstopper.” One of the most unpredictable elements of any rocket launch is weather, and scrubbed launches at Cape Canaveral, caused by lightning storms, low clouds, precipitation, strong winds or other violations of strict weather constraints, are not uncommon. Monday offers a further back-up launch opportunity, a 90-minute window opening at 5.42pm EDT (10.42pm BST), but beyond that engineers would look at rolling back the rocket to the space centre’s giant vehicle assembly building for maintenance that could not be performed at the launchpad. Bill Nelson, the head of Nasa and a former space shuttle astronaut, said the entire craft, from propulsion systems to Orion’s heat shield that must withstand 2,800C (5,000F) temperatures at re-entry, would be heavily “stress-tested” to make sure it was safe for human spaceflight. Ultimately, Nasa is aiming to land humans on Mars around the middle of the next decade, having tested the hardware and systems needed for long-duration spaceflight, including a moonbase, during the Artemis missions. “This is an extremely complicated machine and system. Millions of parts,” Nelson told reporters at Cape Canaveral. “There are, in fact, risks. But are those risks acceptable? I leave that to the experts. My role is to remind them you don’t take any chances that are not acceptable risk.” The cost of the Artemis programme, which is years behind schedule and billions of dollars over budget, has also raised eyebrows. It will have reached an estimated $93bn (£81bn) by 2025, with each of the first four launches alone costing an “unsustainable” $4.1bn, according to Nasa’s independent inspector general. One of the differences between the 1970s Apollo programme – the final three moon missions of which were cancelled on cost grounds – and Artemis, which remains fully funded, is political willpower, according to John Logsdon, founder of the Space Policy Institute at George Washington University. “For the first time since Apollo, two presidents in a row [Donald Trump and Joe Biden] have agreed that this needs to be done, that’s the goal of returning to the moon,” he said. “There’s political support that has been missing before.”