NASA has called off the maiden launch of its 'mega moon rocket', a brand new spacecraft built for exploration and colonization of the solar system, for the second time this week.

A crowd of 400,000 people turned up to watch the Artemis 1 rocket scheduled launch from the Kennedy Space Center in Florida today (Sept. 3) between 2:17pm to 4:17pm EDT, but, just as on its aborted first attempt on Monday (Aug. 29), the uncrewed flight was foiled by technical issues and poor weather conditions.

The rocket's first attempt was scrubbed because engineers were unable to cool one of the rocket's four core stage RS-25 engines down to a safe temperature in time for liftoff. NASA declared that it had fixed the problem, which it blamed on a faulty temperature sensor that incorrectly reported the temperature inside the engine as being much higher, and much further from flight-ready, than it actually was.

Related: Lightning strikes Artemis I mission's 'Mega Moon rocket' launch pad during tests

But this morning, as the rocket was being loaded with the first of its fuel — liquid hydrogen cooled to minus 420 Fahrenheit (minus 250 Celsius) — an alarm sounded, alerting engineers to a gap in the seal of one of the rocket's engines through which the fuel was leaking out. Engineers tried and failed to plug the leak three times, NASA said.

"The Artemis 1 mission to the Moon has been postponed," NASA wrote on Twitter. "Teams attempted to fix an issue related to a leak in the hardware transferring fuel into the rocket, but were unsuccessful."

NASA has yet to announce the launch window for the rocket's third attempt, but said in an announcement on Tuesday (Aug 30.) that another attempt could be made as early as Monday (Sept. 5).

The giant rocket — consisting of the six-person Orion capsule perched atop the 30-story Space Launch System (SLS) 'mega moon rocket' — has been preparing to embark on the first of two test journeys that will pave the way for a human moon landing in 2026, marking humanity's return to the moon for the first time since 1972 and signaling NASA's intent to establish a long-term presence there.

Orion is planned to make two fly-bys of the moon 62 miles (100 kilometers) above the lunar surface, zipping as far out as 40,000 miles (64,000 km) beyond the moon before returning to Earth 38 days after launch.

Stowed aboard Orion are three mannequins that NASA will use to test radiation and heat levels during the flight. A Snoopy soft toy is also along for the ride, floating around inside the capsule as a zero-gravity indicator.

When Orion comes back, it is set to return hotter and faster than any space vehicle ever has, heating up to 5,000 F (2,800 C) as it enters Earth's atmosphere at 32 times the speed of sound. This will put the capsule's ablative heat shield to the test, which, alongside the craft's parachute, will use air friction to slow Orion down to just 20 mph (32.2 km/h), after which it should plop down safely in the Pacific Ocean off the coast of Baja California, Mexico, ready for retrieval.

The flight will be followed by Artemis 2 and Artemis 3 in 2024 and 2025/2026 respectively. Artemis 2 will make the same journey as Artemis 1, but with a four-person human crew, and Artemis 3 will send the first woman and the first person of color to land on the moon's south pole.

Speaking to BBC Radio 4 before the launch, NASA Administrator Bill Nelson said that he was sure that Monday's technical problems had been resolved and that the rocket would fly.

Nasa engineers "have checked it, as we say in the South of the United States, from 'gizzard to gizzard,'" Nelson said. "They're very confident and so therefore I am very confident."

But despite this, Nelson said that a lot of the rocket's other core components had yet to be fully tested.

"The whole rocket is new, the heat shield has to work," Nelson said "We're gonna stress it and test it in a way we'd never do with humans on top of it. But that's the purpose of a test flight. I'm very confident and if there are anomalies or mistakes or unexpected events — that's part of a test flight."

NASA is banking heavily on a successful mission for Artemis 1, which has come under scrutiny for a price-tag that has ballooned to eye-watering levels. The program, which began in 2017, has already cost more than $40 billion to develop and is projected to knock U.S. taxpayers back by $93 billion by the end of 2025, according to the office of NASA inspector general Paul Martin — the space agency's internal auditor.

"Given our estimate of a $4.1 billion per-launch cost of the SLS/Orion system for at least the first four Artemis missions, NASA must accelerate its efforts to identify ways to make its Artemis-related programs more affordable," Martin said at a March 1 testimony before the House Subcommittee on Space and Aeronautics. "Otherwise, relying on such an expensive single-use, heavy-lift rocket system will, in our judgment, inhibit if not derail NASA's ability to sustain its long-term human exploration goals of the moon and Mars."

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Despite the expense, NASA insists that the program is worth it, as it will spur technological innovation and be a crucial next step in humanity's exploration of the cosmos.

"This time we're going not just to touchdown [on the Moon] and leave after a few hours or a few days — we're going back to learn, to live, to work, to explore, to determine is there water; therefore on the [moon's] south pole that would mean we have rocket fuel, we have a gas station up there," Nelson said. "This time we're going to learn how to live in that hostile environment for long periods of time, all with the purpose that we're going to Mars."

Luca Parmitano, a European Space Agency astronaut, wrote on Twitter that such technical issues routinely occur with NASA launches, and that the rocket would still launch eventually.

"A bit of perspective: 11 [NASA] Shuttles had to be rolled back to fix something. 2 of them had to be rolled back twice," he wrote. “When Artemis 1 flies, nobody will remember the delays — had something gone wrong today, however, we'd have remembered it for a long time. So: go Artemis!”

Originally published on Live Science.