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Artemis 1 won't fly today after all.

CAPE CANAVERAL, Fla. — An engine cooling issue on NASA's giant new rocket for deep-space exploration forced the agency to call off the booster's much-anticipated launch debut early Monday (Aug. 29). 

NASA had mostly fueled its first Space Launch System (SLS) megarocket to launch the Artemis 1 moon mission on Monday when launch controllers were unable to chill one of the four main engines to the temperatures needed to handle its super-cold propellant. The issue stalled plans to launch the SLS rocket and its uncrewed Orion spacecraft on an ambitious 42-day test flight around the moon. Liftoff was scheduled for 8:33 a.m. EDT (1233 GMT).

Chilling the SLS rocket's engines before flowing cryogenic liquid hydrogen and liquid oxygen through them is a required step before the rocket can launch, NASA officials said. While three of the engines passed that test, Engine No. 3 did not, despite troubleshooting efforts.

Related: NASA's Artemis 1 moon mission: Live updates More: 10 wild facts about the Artemis 1 moon mission

"Launch controllers condition the engines by increasing pressure on the core stage tanks to bleed some of the cryogenic propellant to the engines to get them to the proper temperature range to start them," NASA officials said in a statement. "Engine 3 is not properly being conditioned through the bleed process, and engineers are troubleshooting."

All four of these engines flew on NASA's space shuttle program of reusable vehicles.

According to NASA spokesperson Derrol Nail, the engine conditioning was not something that the team was able to verify during the "wet dress rehearsal" process that concluded in June.

"This is something they wanted to test during Wet Dress 4 but were unable to," Nail said. "So this was the first opportunity for the team to see this live in action. It's a particularly tricky issue even going in to get that temperature dialed in, according to engineers."

The Engine No. 3 conditioning issue cropped up as NASA worked through a series of glitches during the countdown, including a liquid hydrogen leak early in the fueling process and a possible crack in a part of core booster known as the intertank flange, which connects the SLS's giant liquid hydrogen and liquid oxygen tanks. The tanks can hold a combined 730,000 gallons (3.3 million liters) of propellant.

"The flanges are connection joints that function like a seam on a shirt, are affixed at the top and bottom of the intertank so the two tanks can be attached to it," NASA said in the update.

NASA engineers found that the crack was actually in the insulating foam on the flange, not in the rocket's metal structure. "That ice that formed is essentially air that's being chilled by the tank that gets trapped inside of a crack in the foam but not the actual tank," Nail said. 

Nail added that NASA personnel had seen similar cracks in the foam when it was used on the space shuttles before their retirement in 2011.

The Engine No. 3 problems and the feared crack followed concerns about a liquid hydrogen leak in the rocket. The leak during the fueling process appeared similar to one that occurred during an SLS fueling test earlier this year, Nail noted. But NASA officials were not quick to judge. 

"Although a similar issue was identified in an earlier wet dress rehearsal, it may not necessarily be the same cause," NASA officials wrote in a subsequent update (opens in new tab) .  

NASA stopped and restarted the flow of liquid hydrogen into the tank in an attempt to verify the leak and even proceeded with fueling the 322-foot-tall (98 meters) rocket's upper stage while engineers worked the issue.

Even before fueling the SLS rocket, NASA faced challenges during Monday's launch countdown. Offshore storms and lightning delayed fueling of the SLS rocket by nearly an hour, forcing launch controllers to work to catch up for lost time. 

With NASA unable to launch today, the agency could try for one of at least two back-up days on which to fly Artemis 1 on its mission to the moon. If the agency solves the Engine No. 3 issue, it could try to launch again Friday (Sept. 2) or Sept. 5, weather permitting. If NASA cannot launch by Sept. 5, its next launch try would likely be in October, mission managers have said. Launch opportunities are limited by the stage of the moon and lighting conditions upon reentry, among other considerations.

"The earliest opportunity, depending on what happens with this engine bleed, would be Sept. 2," Nail said. "However, we will await a determination of what the plan is to go forward."

Artemis 1 will send an uncrewed Orion capsule to lunar orbit and back, on a mission that will take six weeks from liftoff to splashdown. It will be the first flight for the long-delayed SLS and the second for Orion, which made a brief trip to Earth orbit back in 2014. (Orion rode atop a United Launch Alliance Delta IV Heavy rocket on that uncrewed test flight.)

— NASA's Artemis program of lunar exploration 

— Artemis 1: Going back to the moon 

— 10 wild facts about the Artemis 1 moon mission 

Artemis 1 will also be the first mission for NASA's Artemis program, which aims to establish a long-term, sustainable human presence on and around the moon. If all goes well with Artemis 1, NASA will be clear to start gearing up for Artemis 2, which will send astronauts on a journey around the moon.

NASA is targeting 2024 for the Artemis 2 liftoff and 2025 or 2026 for Artemis 3, which will land astronauts on the moon for the first time since the final Apollo mission in 1972.

Editor's note: Follow our Artemis 1 mission live updates page for the latest on Artemis 1 mission news. Space.com Spaceflight Editor Mike Wall contributed to this report from San Francisco.

Email Tariq Malik at tmalik@space.com or follow him @tariqjmalik (opens in new tab) . Follow us @Spacedotcom (opens in new tab) , Facebook (opens in new tab) and Instagram (opens in new tab) . 

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Tariq is the Editor-in-Chief of Space.com and joined the team in 2001, first as an intern and staff writer, and later as an editor. He covers human spaceflight, exploration and space science, as well as skywatching and entertainment. He became Space.com's Managing Editor in 2009 and Editor-in-Chief in 2019. Before joining Space.com, Tariq was a staff reporter for The Los Angeles Times covering education and city beats in La Habra, Fullerton and Huntington Beach. He is also an Eagle Scout (yes, he has the Space Exploration merit badge) and went to Space Camp four times as a kid and a fifth time as an adult. He has journalism degrees from the University of Southern California and New York University. To see his latest project, you can follow Tariq on Twitter.

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