By Jackie Wattles, CNN
(CNN) — If all had gone according to plan, four astronauts might have been returning just this week from a history-making, 10-day slingshot trip around the moon.
Instead, NASA’s engineers have been grappling with the rocket and fuel meant to propel the mission, called Artemis II, troubleshooting an all-too-familiar problem.
Just a few hours into a pre-launch test called a wet dress rehearsal in early February, launch controllers found that enough super-chilled liquid hydrogen fuel was leaking at the launchpad to prompt safety concerns. The hydrogen leaks kept cropping up, forcing NASA to halt fuel flow to the rocket multiple times.
The issue ultimately left the space agency unable to complete the full test and led to more than a week of investigations and repairs.
If pesky hydrogen leaks and a delayed moon mission evoke a sense of déjà vu, it may be because NASA has been through this before.
Liftoff of an uncrewed test flight around the moon in 2022, called Artemis I, was delayed several times then nearly thwarted by similar hydrogen seepage before a team of jumpsuit-clad NASA workers swept in at the 11th hour and manually fixed a leaky valve. There are also records of engineers struggling with similar issues throughout the Space Shuttle program, which ran from 1981 to 2011.
Leaks are a major concern on the ground: Hydrogen is very easy to ignite and energetic, meaning that too much of it in one area poses the risk of a catastrophic explosion.
So as launch controllers navigate another “wet dress” rehearsal Thursday, the question remains: Why does NASA keep using this notoriously fickle fuel?
A tiny molecule with a powerful punch
Engineers pioneered the use of hydrogen as rocket fuel in the mid-20th Century before it was used for the Apollo moon rockets — and most of the launch vehicles that have opted for the fuel since have also wrestled with leaks.
For example, the Vulcan Centaur rocket, which is produced by US-based military contractor United Launch Alliance and builds on decades of legacy technology, uses hydrogen to power the upper part of its rocket. And in 2023, a fuel leak caused a fireball explosion during Vulcan Centaur testing in Alabama, damaging nearby infrastructure and delaying the rocket’s inaugural launch.
Hydrogen’s leaky tendencies can be attributed to the fact that it’s the lightest element in the universe. It “tends to find its way out of things you want to try to contain it in,” said Adam Swanger, a senior principal investigator and cryogenics research engineer at NASA’s Kennedy Space Center. “And it has very low density.”
To put that in perspective, hydrogen is roughly 14 times lighter than the air on Earth. But the same properties that make hydrogen so difficult to contain also make it an ideal rocket fuel.
“The low density is good for performance,” Swanger told CNN. “So there’s kind of a trade-off there.”
When selecting fuel for a rocket, the most important consideration is a concept called “specific impulse,” often abbreviated as Isp. It’s a measure of how much thrust — or force — a rocket engine can generate with a set amount