NASA Begins Artemis II Wet Dress Rehearsal

- The full wet dress rehearsal is a nearly 50-hour process that simulates the entire launch countdown, including two runs through the final ten minutes, known as the terminal count. This second attempt was scheduled after a previous rehearsal in early February was scrubbed due to a liquid hydrogen (LH2) leak at the tail service mast umbilical. - During the test, launch controllers load over 700,000 gallons of cryogenic propellants—liquid hydrogen chilled to -423°F and liquid oxygen chilled to -297°F—into the Space Launch System (SLS) rocket's core stage and Interim Cryogenic Propulsion Stage (ICPS). The process involves a "chilldown" to condition the lines, followed by a "slow fill" and then a "fast fill." - A significant test objective is to practice countdown holds and recycling. During the terminal count, the team intentionally pauses the clock at T-1 minute and 30 seconds for up to three minutes, and again at T-33 seconds, before resetting to T-10 minutes for a second run. This validates procedures for handling potential technical or weather-related launch scrubs. - While the four Artemis II astronauts—Reid Wiseman, Victor Glover, Christina Koch, and Jeremy Hansen—were present in the Launch Control Center, they did not board the Orion capsule for this test. However, a closeout crew practiced all launch day procedures at the pad, including securing the Orion crew module hatch and the launch abort system hatch. - This rehearsal is the final major test before NASA sets a definitive launch date, with the earliest opportunity currently targeted for March 6, 2026. Following a successful rehearsal, the data is reviewed, and teams will service the flight termination system on the solid rocket boosters and core stage to meet Eastern Range safety requirements. - The rehearsal also tests the response to anomalies. During this test, the team experienced a temporary loss of ground communications, which required switching to a backup system before the primary was restored. A booster avionics system voltage anomaly also caused a brief pause in the terminal countdown. - Engineers closely monitored for hydrogen leaks, a persistent issue during the Artemis I campaign and the first Artemis II rehearsal. Hydrogen, the smallest molecule, is a highly efficient but notoriously leaky propellant. For this test, a small, persistent leak was detected but remained within acceptable safety limits. - The successful completion of fueling and the dual terminal count simulations are critical milestones that clear the path for the 10-day crewed mission, which will send astronauts on a flyby of the Moon, traveling farther into space than any human has before.