A serious and unmanageable hydrogen leak prevented NASA from launching its SLS rocket on Saturday. Teams continue to evaluate the reason for the second failed launch attempt, but NASA says the rocket will have to return to the Vehicle Assembly Building to undergo safety checks.

For rocket scientists, hydrogen is an important propellant, but it’s also a serious pain in the ass. It’s the tiniest molecule in the universe, and as such, it has a propensity to leak out through tiny gaps and cracks. Hydrogen leaks bedeviled the Space Shuttle program, and now SLS—a rocket heavily modeled after the Shuttle launch system—is likewise experiencing problems with hydrogen leaks.

The most recent leak, which prevented the Space Launch System from blasting off today, was more severe than the one experienced during the first launch attempt on Monday, Mike Sarafin, Artemis mission manager, told reporters during a press briefing held earlier today. “This was not a manageable leak,” he said, saying pre-planned procedures to fix the problem failed to work. Ground crews were able to resolve a hydrogen leak on Monday, but the techniques applied today “were not working in our favor,” Sarafin said. Three different attempts were made today to fix the problem. None worked. Today’s launch was supposed to happen within a two-hour window that opened at 2:17 p.m. ET.

Jim Free, associate administrator for Exploration Systems Development Mission Directorate at NASA said that the current launch period, which ends on Tuesday, September 6, is now “off the table.” The next launch attempt for SLS could happen within a period that runs from October 17 to 31. “We do not launch until we think it’s right,” said NASA administrator Bill Nelson during today’s briefing. He reminded reporters that the Shuttle got sent back to the Vehicle Assembly Building (VAB) on more than 20 occasions.

NASA officials are currently devising a plan to move forward with the rocket. Repairs could be done either while the rocket remains on Launch Pad 39B at Kennedy Space Center or inside the VAB nearby. During tanking on Saturday, a line in the 8-inch quick disconnect appeared to have been affected by an inadvertent over-pressurization. Sarfin said this might be a cause, and possibly the root cause, of the hydrogen leak. At today’s press briefing, Free said the large hydrogen leak at the quick disconnect was different than the one experienced on Monday.