John Honeycutt, head of NASA’s Artemis II mission management team, said the choice to loosen the safety threshold between Artemis I and Artemis II was driven by test results.
“The SLS program developed a test campaign that examined that cavity, the cavity’s properties, the purge in the cavity … and they introduced hydrogen to determine when it would ignite, and at 16 percent, it did not,” said Honeycutt, who served as NASA’s SLS program manager before moving to his current role.
When mixed with air at high concentrations, hydrogen is explosive — which is why it’s an effective rocket propellant. But it’s also famously hard to contain. Molecular hydrogen is the tiniest molecule, so it can escape through leak paths, and it creates a materials challenge for seals because liquefied hydrogen is chilled to minus 423 degrees Fahrenheit (minus 253 degrees Celsius).
In practice, NASA used the three-year gap between Artemis I and Artemis II to become comfortable tolerating a larger hydrogen leak rather than repairing the leaks themselves. Isaacman said that approach will change before Artemis III, which is also likely at least three years away.
“I will say near-conclusively for Artemis III, we will cryoproof the vehicle before it gets to the pad, and the propellant loading interfaces we are troubleshooting will be redesigned,” Isaacman wrote.
Isaacman became NASA’s administrator in December and has criticized the SLS program’s steep cost—estimated by NASA’s inspector general at more than $2 billion per rocket—along with the launch vehicle’s slow flight rate.
NASA’s spending on the rocket’s ground systems at Kennedy Space Center is similarly massive. Nearly $900 million was spent on Artemis ground-support infrastructure in 2024 alone. Much of that funding went toward building a new launch platform for an upgraded version of the Space Launch System that may never fly.
That combination makes each SLS rocket a golden egg — a bespoke vehicle that must be handled carefully because it’s too costly to replace. NASA and Boeing, the prime contractor for the SLS core stage, never produced a full-size test article of the core stage. Right now there’s no way to fully validate the cryogenic interactions between the core stage and ground equipment until the complete rocket is assembled on the launch pad.
Existing law requires NASA continue flying the SLS rocket through the Artemis V mission. Isaacman wrote that the Artemis architecture “will continue to evolve as we learn more and as industry capabilities mature.” In other words, NASA plans to incorporate newer, cheaper, reusable rockets into the Artemis program.
The next set of launch opportunities for the Artemis II mission opens March 3. If the mission doesn’t lift off in March, NASA will roll the rocket back to the Vehicle Assembly Building to service its flight-termination system. Additional launch dates are available in April and May.
“There’s still a lot of work ahead to prepare for this historic mission,” Isaacman wrote. “We will not launch unless we are ready and the safety of our astronauts will remain the highest priority. We will keep everyone informed as NASA prepares to return to the Moon.”
