NASA engineers held the countdown at T-40 minutes while troubleshooting for more than an hour. Finally, launch director Charlie Blackwell-Thompson called the attempt a scrub. At a press conference the following day, members of the Artemis team suggested the apparent engine issue might actually have been a sign of a dodgy temperature sensor. “The way the sensor is behaving does not line up with the physics of the situation,” said John Honeycutt, the SLS program manager.

The launch was then pushed back to this weekend, with countdown procedures starting up again early Saturday morning. Anticipating challenges with the propellants, they began the chill-down process, including the kickstart test, about 45 minutes earlier during the countdown procedures. The launch team and weather officer confirmed that the weather was amenable to launch, despite a few intermittent rain showers. They began filling the big orange fuel tank with more than 700,000 gallons of liquid hydrogen and liquid oxygen, supercooled to a frigid -423 and -297 degrees Fahrenheit.

But that’s when the hydrogen leak arose, after the oxygen had been mostly fueled up. “Hydrogen’s difficult to work with,” said Jim Free, associate administrator at NASA headquarters, during the post-scrub press conference. The leak seems to stem from a seal in the 8-
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