NASA’s next Artemis II countdown test hits a new snag—what it means and why it’s fixable

Background

Artemis II is NASA’s first crewed flight of the Artemis program, a roughly 10-day trip sending four astronauts around the Moon and back to Earth in the Orion spacecraft. The mission will fly on the Space Launch System (SLS), a heavy-lift rocket derived from shuttle-era propulsion and built to loft Orion on a translunar trajectory. While the astronauts won’t land on the Moon—that’s Artemis III’s job—Artemis II is the critical shakedown of NASA’s deep-space transportation system with people on board.

Before committing a crew to the vehicle, NASA has been performing a series of pad rehearsals at Kennedy Space Center’s Launch Complex 39B. These tests are called wet dress rehearsals (WDRs): the team loads the core stage and upper stage with super-cold liquid hydrogen (LH2) and liquid oxygen (LOX), runs the countdown into terminal phases, and then safes and detanks the vehicle. It’s the only way to validate ground equipment, procedures, software, and rocket hardware under realistic conditions short of lighting the engines.

Cryogenic fueling is notoriously unforgiving. Hydrogen’s tiny molecules find gaps that other fluids can’t, and the thermal shock of chilling warm metal down to hundreds of degrees below zero can open or close microscopic leak paths. The shuttle program wrestled with hydrogen leaks at umbilicals for years; SLS inherits both the benefits and the challenges of that heritage.

What happened

NASA’s latest Artemis II WDR showed meaningful progress on leak control compared with the first attempt, but it also produced an unwelcome twist: a different hardware issue appeared late in operations, forcing the team to stand down and plan corrective work before trying another full-up countdown.

Here’s what is known and what can be reasonably inferred from NASA’s public updates and on-the-record comments:

• The agency reported that hydrogen readings during this second rehearsal were substantially lower than during the first WDR, a sign that seal replacements, torque adjustments, and procedural tweaks made after the earlier attempt are paying off.
• The team proceeded deeper into the loading sequence and was able to gather far more data across multiple flow regimes—slow fill, fast fill, and topping—suggesting greater stability in the propellant systems and ground support equipment.
• A separate issue then cropped up, unrelated to the earlier hydrogen leak signatures. NASA characterized it as a problem that must be corrected before the next attempt. While NASA did not publish every technical detail, the description points to a specific component or interface in the cryogenic or pneumatic plumbing (for example, a valve, quick-disconnect, or sensor pathway) that did not behave to spec under full cryogenic conditions.
• Because the problem appeared under realistic thermal and pressure loads, it likely could not have been fully exposed in ambient checks. That is exactly why wet dress testing exists: to surface latent faults with the rocket and its mobile launcher under flight-like conditions.

The team will now perform a fault tree analysis, review data from temperature and pressure sensors across the affected circuits, and decide whether the fix can be executed at the pad or requires rolling SLS back to the Vehicle Assembly Building (VAB). A pad-side swap of a line-replaceable unit might take days; a rollback, depending on schedules and subsystem scope, can add weeks.

Despite the setback, the data trending remains encouraging. Lower hydrogen signatures than the first WDR mean the mitigation steps are working, and the new issue appears to be localized rather than systemic. That’s good news for overall reliability once the component is repaired or replaced and verified with another countdown run.

Why hydrogen is so hard—and why that matters here

A few physics and operations realities frame the situation:

• Hydrogen is slippery. Its molecules are so small that even tiny imperfections in seals or mating surfaces can become leak paths, especially when metal contracts under cryogenic loads.
• Temperature swings matter. Going from Florida’s ambient conditions to 20 K for liquid hydrogen and 90 K for liquid oxygen induces mechanical stresses. Gaps open and close, O-rings change behavior, and valves can stick unless precisely conditioned.
• Leak limits are strict for safety. The lower flammability limit for hydrogen in air is about 4% by volume. NASA sets tight thresholds and monitors multiple sniffers and line sensors. Readings that are acceptable in one phase (e.g., initial chilldown) can be unacceptable in another (e.g., when the ground vent is closed and the system is pressurizing).
• Procedure is part of the hardware. How quickly you cool down a line, the order in which you open valves, and the target temperatures and pressures all influence whether a seal weeps or stays tight. Shuttle taught NASA to “load gently” when needed; SLS has adopted similar profile options.

The upshot: recurring leak hunting isn’t a sign of negligence; it is the norm in the cryogenic world. The work is to make the leaks small, predictable, and within safety and launch commit criteria—and to fix any out-of-family behavior that appears late in the sequence.

Key takeaways

• The second Artemis II WDR made real progress. Hydrogen readings improved compared to the first attempt, allowing the team to advance further in the script and collect higher-quality data.
• A new, specific hardware issue emerged and must be corrected before another full countdown. The problem appears localized, the sort of fault that becomes apparent only under cryogenic, high-flow conditions.
• NASA is following a textbook test-fix-test cycle. This is the same pattern that ultimately cleared SLS for its Artemis I launch after multiple rehearsals and targeted seal replacements.
• Schedule impacts are possible but not mission-defining. Whether the team fixes at the pad or rolls back will drive near-term timelines. The broader Artemis II critical path still runs through verification of Orion life-support upgrades, ground systems readiness, and crew training.
• Lessons here also feed forward to Artemis III. Even though Artemis III’s critical path is dominated by the lunar lander and new suits, NASA wants boring, predictable tanking on SLS. Each rehearsal tightens that playbook.

What to watch next

• NASA’s fault isolation and repair plan. The agency will share whether the work is a pad-side component swap, an umbilical seal servicing, or a more involved task necessitating a rollback.
• The next rehearsal goalposts. Expect NASA to set a new target to run deeper into terminal count—potentially to the point just prior to engine start sequence handover—to prove the full stack under pressure.
• Procedural refinements. Look for mentions of adjusted chilldown profiles, modified valve timings, or updated leak criteria at specific phases. These small changes can yield big gains in leak control.
• Data on leak rates. While raw numbers are rarely published in detail, NASA may describe trends—e.g., “below constraint,” “stable and decreasing,” or “spiked during replenish”—that indicate whether they’re converging on a robust load.
• Range and weather windows. Florida’s spring and summer weather is a wild card. Lightning and high winds can constrain both testing and rollbacks, padding the schedule.
• Integration with crew timelines. The rehearsal cadence has to sync with astronaut training milestones and Orion processing. NASA will guard crew readiness margins closely.

Deeper context: How SLS fueling actually unfolds

SLS core stage loading is a ballet involving more than 700,000 gallons of cryogenic propellants across the rocket and the ground systems on Mobile Launcher-1 (ML-1):

• Chilldown: Before any liquid is introduced, lines are cooled with small flows to prevent flash boiling and violent thermal shocks.
• Slow fill: Valves crack open to admit a trickle of cryogenic fluid, continuing to cool hardware and avoid rapid pressure excursions.
• Fast fill: Once temperatures stabilize, flow ramps up dramatically to load the bulk of the propellant.
• Replenish and topping: As tanks boil off, the system periodically refills to keep levels within tight bands. This phase can be leak-sensitive because venting profiles and pressures change, closing some paths that were previously relieving.
• Pre-pressurization checks: Near terminal count, the system transitions from ground-based venting to flight-like conditions. This is where any marginal hardware often reveals itself.

At each step, leak detectors and temperature/pressure sensors watch dozens of points around umbilicals such as the Tail Service Mast Umbilicals (TSMUs), the Core Stage Intertank Umbilical, and the forward umbilicals near Orion. A single out-of-family reading can trigger a hold if it threatens safety or violates launch commit criteria.

Why this isn’t the end of the world for Artemis II

If you followed Artemis I, you’ve seen this movie. In 2022, SLS required multiple wet dress attempts before NASA was comfortable with the tanking environment. The agency swapped seals on a quick-disconnect, modified loading procedures to be gentler, and ultimately launched successfully. That experience established a playbook: test under cryo, find problems, fix surgically, and re-test until the system behaves predictably.

Two factors limit the long-term impact here:

• The trend line is improving. Lower hydrogen signatures on the second rehearsal indicate the mitigations are working.
• The new fault is actionable. Discrete hardware anomalies—valves, seals, sensors—are fixable. They do not generally imply a redesign of the entire ground system.

Could there be more surprises? Of course. That’s the nature of complex launch systems. The goal is to make each round of surprises smaller, rarer, and quicker to resolve.

The broader Artemis chessboard

It’s also worth placing the rehearsal in the context of Artemis as a whole:

• Artemis II’s pacing items are not just on the pad. Orion’s life support, batteries, and avionics still must clear crewed-flight verification gates. Those remain equally important to schedule.
• Artemis III’s pacing items lie largely outside SLS. The human landing system (a lunar-optimized Starship variant) and next-generation lunar spacesuits are the dominant drivers. That doesn’t give SLS a free pass—but it does mean incremental ground-system tuning is unlikely to be the long pole for the landing mission.
• ML-2 and Block 1B loom in the background. While Artemis II and III use SLS Block 1 on Mobile Launcher-1, NASA is already developing the taller ML-2 for the more powerful Block 1B. Lessons learned on hydrogen management today will flow directly into those designs and procedures.

Bottom line

NASA would obviously prefer a clean countdown with zero surprises. But a rehearsal that both improves leak behavior and exposes a new, fixable fault is still a step forward. The next milestone is a clear plan to service the offending component, repeat the test, and press deeper into terminal count. If the trend continues—lower hydrogen readings, no fresh anomalies—that will build the confidence needed to set a launch window for Artemis II.

FAQ

• What is a wet dress rehearsal?

  • It’s a full-fidelity practice countdown in which NASA loads the rocket with cryogenic propellants and runs through the launch sequence down to moments before engine start. No ignition occurs; the vehicle is then safed and drained.

• Why does NASA use liquid hydrogen if it leaks so easily?

  • Hydrogen is challenging, but it offers excellent performance. Paired with liquid oxygen in SLS’s RS-25 engines, it delivers very high efficiency, helping lift Orion and its massive launch abort system to orbit with enough energy for the Moon.

• Are these leaks dangerous to the crew?

  • Leak thresholds are conservative, and astronauts won’t be on board during fueling. Multiple sensors, automatic holds, and strict safety rules control the risk. NASA will not fuel or launch if it can’t meet its safety criteria.

• Does this delay Artemis II?

  • It can impact near-term rehearsal dates, especially if a rollback is needed. Whether it shifts the crewed launch date depends on the scope of the fix and how it lines up with other Artemis II readiness items.

• Didn’t SLS have fueling problems before?

  • Yes. During Artemis I rehearsals, hydrogen leaks at umbilical interfaces forced several fixes and procedure changes. After those updates, SLS launched successfully. The current situation is similar: find the issue, fix it, and re-test.

• What about Artemis III?

  • NASA leadership has emphasized that they intend to iron out fueling reliability well before the lunar landing mission. Still, Artemis III’s critical path is dominated by the lunar lander and suits, not SLS tanking.

Source & original reading: https://arstechnica.com/space/2026/02/nasa-chief-vows-to-solve-sls-rocket-fueling-issues-before-artemis-iii/