NASA is tackling fresh fueling issues with its Space Launch System (SLS) rocket ahead of the Artemis II mission, the first crewed flight of the program that will send astronauts around the Moon. The agency aims to resolve these technical problems before attempting the second full countdown rehearsal next week at Kennedy Space Center in Florida.

Artemis II follows a practice countdown earlier this month that was cut short due to a hydrogen fuel leak. NASA engineers traced the problem to ground support equipment, specifically the connections where fueling lines attach to the rocket’s core stage. The leak affected the Tail Service Mast Umbilicals (TSMUs), the gray towers that deliver liquid hydrogen and oxygen into the rocket before launch.

During Thursday’s “confidence test,” NASA partially filled the core stage with liquid hydrogen to check the recently replaced seals. While the test ended prematurely because of a filter suspected to reduce fuel flow, engineers collected valuable data. NASA Administrator Jared Isaacman noted the leak rates were “materially lower” than during the first wet dress rehearsal, signaling progress even though the rocket wasn’t fully stressed.

Hydrogen, a highly volatile fuel, is difficult to contain. Its tiny molecules easily escape, and the super-cold temperature required for liquid hydrogen makes it challenging to seal. In a safety update, NASA raised the acceptable hydrogen concentration near the fueling connections from 4% to 16%, based on new testing that showed the higher limit was still safe.

Artemis II will carry four astronauts around the far side of the Moon on a nearly 10-day mission before returning them to Earth. Despite the setbacks, NASA officials emphasize safety as the top priority. “We will not launch unless we are ready,” Isaacman said, noting that engineers are working to redesign the fueling interfaces for the next crewed mission, Artemis III, which aims to land astronauts on the Moon.

The Artemis program’s SLS rockets are extremely expensive and unique, with costs exceeding $2 billion per vehicle. NASA and Boeing, the core stage contractor, never built a full-scale test model, meaning many fueling and mechanical challenges can only be fully assessed on the launch pad. The agency also spent $900 million in 2024 on ground support systems for Artemis, including a new launch platform.

The next launch windows for Artemis II open on March 3, with backup opportunities in April and May. If issues persist, the rocket may be rolled back to the Vehicle Assembly Building for additional system checks. NASA’s Artemis team remains committed to overcoming these challenges to safely return humans to lunar orbit for the first time in more than 50 years.