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The uncrewed Artemis I mission will get another attempt at launching on a journey around the moon on Saturday.
The two-hour launch window opens at 2:17 p.m. ET on September 3.
After the launch was scrubbed Monday morning, the launch team spent the remainder of the day evaluating data gathered during the attempt. Mission managers shared an update Tuesday evening.
The Artemis I stack, which includes the Space Launch System rocket and Orion spacecraft, continues to sit on Launchpad 39B at Kennedy Space Center in Florida.
One of the rocket’s four RS-25 engines, identified as engine #3, could not reach the proper temperature range that is required for the engine to start at liftoff.
The engines need to be thermally conditioned before supercold propellant flows through them before liftoff. To prevent the engines from experiencing any temperature shocks, the launch controllers increase the pressure of the core stage liquid hydrogen tank to send a little bit of the liquid hydrogen to the engines. This is known as a “bleed.”
The liquid hydrogen is about minus 423 degrees Fahrenheit (minus 252 degrees Celsius).
Engine #3 was probably about 30 to 40 degrees warmer than the other engines, which reached about minus 410 degrees Fahrenheit (minus 245 degrees Fahrenheit), said John Honeycutt, manager of the Space Launch System Program at NASA’s Marshall Space Flight Center in Alabama.
Mission managers suspected that the engine #3 issue was actually a problem with the bleed system, rather than the actual engine. A faulty sensor may be providing an incorrect reading of the engine temperature, Honeycutt said.
“The way the sensor is behaving doesn’t line up with the physics of the situation,” Honeycutt said.
The team plans to begin the bleed 30 to 45 minutes earlier in the countdown than it occurred on Monday and monitor the engine temperature during the bleed.
“Coming into yesterday’s attempt, we said that if we couldn’t thermally condition the engines, we’re not going to launch,” Sarafin said. “That’s the same posture that we’re going into Saturday.”
Removing and replacing the sensor would be tricky at the launchpad, so the only alternative is rolling it back into the Vehicle Assembly Building for servicing, said Charlie Blackwell-Thompson, Artemis launch director at NASA’s Exploration Ground Systems Program.
Several other issues, like storms, a leak on an 8-inch line used to fill and drain the rocket core stage’s liquid hydrogen and a hydrogen leak from a vent valve on the core stage’s intertank also caused delays Monday morning that prevented liftoff during the two-hour launch window.
“”We agreed on what was called option one, which was to operationally change the loading procedure and start our engine chill down earlier. We also agreed to do some work at the pad to address the leak that we saw at the hydrogen tail service mask umbilical,” said Mike Sarafin, Artemis mission manager, NASA Headquarters.
The current forecast for Saturday includes a chance of showers and thunderstorms in the morning and early afternoon hours, so the launch team will keep a close eye on the forecast, said meteorologist Mark Burger, launch weather officer with the US Space Force’s 45th Weather Squadron.
There is a 60% chance of a weather violation during the launch window, Berger said.
There is still a backup opportunity for the Artemis I mission to launch on September 5 as well.
The Artemis I mission is just the beginning of a program that will aim to return humans to the moon and eventually land crewed missions on Mars.
Correction: An earlier version of this story misspelled meteorologist Mark Burger's name.