NASA officials have declared the Artemis 1 moon rocket's most recent "wet dress rehearsal" a success and are hopeful the mission can get off the ground as soon as late August.
The Artemis 1 stack — a Space Launch System (SLS) rocket topped by an Orion capsule — is scheduled to roll back to the Vehicle Assembly Building (VAB) at NASA's Kennedy Space Center (KSC) in Florida on July 1, where the massive vehicle will undergo repairs and preparations for its coming launch.
Artemis 1, the first launch for the SLS, will send an uncrewed Orion on a roughly month-long mission around the moon. The mission has experienced several delays, and most recently the rocket's certification to fly has been held up by incomplete fueling tests — a key part of the wet dress rehearsal, a three-day series of trials designed to gauge a new vehicle's readiness for flight.
The Artemis 1 stack first rolled from the VAB to KSC's Pad 39B in mid-March, to prep for a wet dress rehearsal that began on April 1. But three separate attempts to fill the SLS with cryogenic propellants during that effort failed, sending the stack back to the VAB for repairs on April 25. The most recent wet dress try, which wrapped up on Monday (June 20), didn't go perfectly, but NASA has deemed it good enough to proceed with preparations for launch.
Operators were able to fully fuel SLS for the first time, bringing the launch simulation much further along than any of the attempts in April. A leak from the core stage's engine cooling system "umbilical" line was detected during Monday's fueling test. Closing a bleed valve in the hydrogen's path flow allowed operators to resolve the leak but left engine thermal systems without proper regulation. This would be an issue for an actual launch, but not for wet dress since the engines don’t actually fire.
Knowing the temperature variance would be flagged by the ground launch sequencer during terminal count, operators decided to run a software "mask," which permitted computers in mission control to acknowledge the deviation without flagging it as a reason to halt the clock, according to Phil Weber, senior technical integration manager at KSC. Weber joined other agency officials on a press call Friday (June 24) to discuss the plans for Artemis 1 now that the wet dress is in the rear view mirror.
The software mask allowed the count to continue through to the handoff from the mission control computers to the automated launch sequencer (ALS) aboard the SLS at T-33 seconds, which ultimately terminated the count at T-29 seconds.
"[ALS] was really the prize for us for the day," Weber said during Friday's call. "We expected … it was going to break us out [of the countdown] because the ALS looks for that same measurement, and we don't have the capability to mask it onboard."
It was unclear immediately following the recent wet dress if another one would be required, but mission team members later put that question to rest.
"At this point, we've determined that we have successfully completed the evaluations and required work we intended to complete for the dress rehearsal," Tom Whitmeyer, deputy associate administrator for Common Exploration Systems at NASA headquarters, said on Friday's call. He added that NASA teams now have the "go ahead to proceed" with preparations for Artemis 1's launch.
Before it can be rolled back to the VAB, however, the stack will undergo further maintenance at Pad 39B, including repairs to the quick-disconnect component on the aft SLS umbilical, which was responsible for Monday's hydrogen leak.
There's also one more test technicians need to perform at the pad. Hot-firing the hydraulic power units (HBUs), part of the SLS' solid rocket boosters, was originally part of the wet dress countdown but was omitted when the countdown was aborted. Those tests will be completed by Saturday (June 25), according to Lanham. Following the hot-fire tests, operators will then spend the weekend offloading the HBUs' hydrazine fuel.
Once back in the VAB, NASA officials estimate it'll take six to eight weeks of work to get Artemis 1 ready to roll back to Pad 39B for an actual liftoff. Cliff Lanham, senior vehicle operations manager at KSC, outlined some of the planned maintenance on Friday's call.
Among other tasks, technicians will perform standard vehicle inspections, hydrogen leak repairs, "late-stow" for the payloads flying on Orion, and software loads to the SLS core stage and upper stage. They will also install flight batteries.
"Ultimately, we want to get to our flight termination system testing," Lanham said. "Once that's complete, we'll be able to perform our final inspections in all the volumes of the vehicle and do our closeouts."
After that work is complete, the Artemis 1 stack will roll out from the VAB once again, making the eight to 11-hour crawl back to Pad 39B on July 1. Whitmeyer said on Friday that the late-August launch window for Artemis 1, which opens on Aug. 23 and lasts for one week, is "still on the table."
Editor's note: This story was updated at 3 p.m. EDT on June 28 to clarify that the Artemis 1 team "masked" a temperature-variation issue during the most recent wet dress rehearsal, not a hydrogen leak as originally stated. The leak was resolved by closing a valve, so it was not masked.
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Josh Dinner is Space.com's Content Manager. He is a writer and photographer with a passion for science and space exploration, and has been working the space beat since 2016. Josh has covered the evolution of NASA's commercial spaceflight partnerships, from early Dragon and Cygnus cargo missions to the ongoing development and launches of crewed missions from the Space Coast, as well as NASA science missions and more. He also enjoys building 1:144 scale models of rockets and human-flown spacecraft. Find some of Josh's launch photography on Instagram and his website, and follow him on Twitter, where he mostly posts in haiku.