The first failure was related to a faulty sensor, and the second to a fuel leak when filling the rocket's tanks. It runs on ultra-cold, ultra-volatile liquid oxygen and hydrogen.
NASA has since replaced a seal and modified its procedures to avoid thermal shock as much as possible.
The Orion capsule will be lifted by two boosters and four powerful engines under the core stage, which will detach after only a few minutes.
After a final push from the upper stage, the capsule will be well on its way, taking several days to reach its destination.
Rather than landing on the Moon, it will assume a distant orbit, venturing 40,000 miles (64,000 kilometers) beyond the far side -- further than any other habitable spacecraft so far.
Finally, Orion will embark on the return leg of its journey. When passing through the atmosphere, the capsule's heat shield will need to withstand a temperature half as hot as the Sun's surface.
If takeoff happens Wednesday, the mission will last 25 and a half days, with a splashdown in the Pacific Ocean on December 11.
NASA is banking on a successful mission after developing the SLS rocket for more than a decade. It will have invested more than $90 billion in its new lunar program by the end of 2025, according to a public audit.
Artemis 2 will involve a flyby of the Moon with astronauts in 2024.
Boots on the ground should happen during Artemis 3, no sooner than 2025, with the crew set to include the first woman and first person of color on the Moon.
