NASA is about to take its first baby steps back to the moon. Phase one of the plan, fittingly called Artemis 1, plans to dispatch an uncrewed test flight around the moon and back. It will be the first flight of the Artemis program, which aims to put humans on the moon again by the mid-2020s.

But before Artemis 1 can happen, NASA needs to ensure that its highly touted rocket—known as the Space Launch System (SLS)—is fully operational. That still hasn’t happened; the rocket’s latest dress rehearsal ended prematurely earlier this month. It’s the third in a sequence of unsuccessful practice sessions for the team of engineers.

One of the key problems is that NASA hasn’t been able to pump all of the Artemis rocket’s fuel reserved into its tanks. As NASA engineers tested filling up the tanks on the Kennedy Space Center launch pad, technical issues and leaks prevented the process from being completed. It might sound bad, but this sort of obstacle rings all too familiar in space launches.

“I’m hopeful that there won’t be too many delays, and they’ll figure out what is causing the problems pretty quickly and be able to get back on a better track pretty soon,” says Makena Young, an associate fellow in space at the Center for Strategic and International Studies in Washington, D.C.

[Related: Inside NASA’s messy plan to return to the moon by 2024]

When the Obama administration green-lit the SLS rocket concept a decade ago, NASA said it would be ready for launch by 2016 or 2017. But experimental rockets in this stage of their development are almost expected to have major difficulties. A rocket is a highly complex operation—it’s not good enough to just plop one atop a pad and light it like a firework. A multitude of intricate subsystems have to work together in it for it to help astronauts and other precious cargo reach space.

The pad itself, for instance, is far more than a temporary resting site for a rocket before it takes off. In the case of SLS, it’s a mobile launch tower that rolls onto the launchpad and plays several critical roles.

Sprouting from that toweris a mass of tendrils, called umbilicals, latched onto the rocket. One umbilical allows astronauts to board the rocket when the crew capsule is hundreds of feet in the air. Others act as stabilizers that keep the rocket steady on the launchpad. Still others include cables that provide vital electrical and communications links between the ground and the rocket.

And all of those umbilicals, which operate and separate in different ways, must effortlessly detach at liftoff. That’s a big ask when the object they’re anchored too is a fiery 365-foot-tall beast that can set off car alarms from miles away. “It’s a violent, violent atmosphere for those components to be in,” says Kevin Miller, an engineer at NASA’s Kennedy Space Center. “It’s an interesting environment unlike anything else in the world, and we have to make sure it operates flawlessly.” 

Not all umbilicals attach to the tower of the spacecraft. On the other side of the rocket, two more tendrils rise from the ground and link up near its bottom. One of the chief functions of these Tail Service Mast Umbilicals (TSMUs) is to help fill SLS’s fuel tanks as the rocket sits on the launchpad.

When it finally comes time to fly, the SLS propels itself with two some simple elements: hydrogen and oxygen. Pump them into the same chamber, ignite them, and the subsequent reaction creates water and massive amounts of energy that allow the rocket to push past Earth’s atmosphere and gravitational field. (That’s just the main stage. The SLS also relies on a pair of boosters like crutches to help push it farther up into the sky. These burn aluminum-based solid fuel through an entirely different process.)

But because gases are not very dense, trying to store that hydrogen and oxygen in their room-temperature forms would require fuel tanks far too large to be practical on a rocket. Instead, these elements must be kept in chilled liquid states. That cold is nothing to sneeze at. Oxygen liquifies at -297 degrees Fahrenheit (-183 degrees Celsius). Meanwhile, liquid hydrogen’s boiling point is a more biting -423 degrees F (-253 degrees C).

In its latest test, NASA filled up 49 percent of the liquid oxygen and 5 percent of the liquid hydrogen before technicians spotted a hydrogen leak and halted filling.

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“A small quantity of liquid hydrogen becomes an absolutely huge gaseous hydrogen cloud,” says Miller. That flammable fallout could be disastrous if it’s not fixed before takeoff.

“Many launches have been scrubbed by propellant leaks over the years, either with launch vehicle or ground system hardware,” says Jeff Foust, a space writer who has been following SLS since the project’s inception. Throughout history, numerous space shuttle launches have faced problems with leaks. “These problems are tracked down and fixed, and the vehicles eventually launch,” says Foust.

While circular liquid hydrogen storage tank at Kennedy Space Center launch pad
Each of the liquid hydrogen and liquid oxygen tanks at the launch pad can hold more than 800,000 gallons of propellant. Ben Smegelsky/NASA

But just as the TSMUs need to function properly to fuel the rocket, they also need to pump it flush with nitrogen gas. Doing this purges the rocket’s system of the aforementioned hydrogen, minimizing potential fire hazards. Nitrogen also acts as climate control to keep the rocket’s components at a steady temperature and low humidity—something that’s especially important in Florida’s subtropical climate.

“It keeps all the electronics and such happy when they’re nice and cool and dry,” says Miller.

Still, even at this stage, such errors are expected. Dress rehearsals and other tests are designed to catch finicky problems before they can plague actual launches involving astronauts and pricy space instruments.

[Related: We could live in caves on the moon. What would that be like?]

As for what this means for the future of Artemis launches, the timetable isn’t certain. Successful tests are necessary before the first full uncrewed launch can be schedule. At this rate, Artemis I might not speed off to the moon until June or early July.

If launching Artemis flights to the moon is anything like launching arrows from the bow of its namesake goddess, then it will be another few weeks before NASA knows that the bow works at all. Only after that can the mission lift off.

“It’s not a good thing that it’s being delayed, but it’s a good thing that they’re taking every precaution to make sure that this will be a safe and successful launch,” says Young.

That said, any delays to Artemis 1 will put pressure on the scheduling of Artemis 2: the first crewed mission, which will put three US and one Canadian astronaut in lunar orbit for 10 days. Artemis 2 is currently slated to launch in May 2024, but it will take roughly two years to prepare after Artemis 1. The longer Artemis 1’s launch slips, the longer humans will have to wait to visit the far side of the moon.

Only after that comes phase three: placing boots back in moon dust for the first time since 1972.

Correction (April 27, 2022): The story previously mixed up the percentage levels of liquid oxygen and hydrogen levels that were injected into the SLS rocket during NASA’s most recent wet dress rehearsal. It also incorrectly stated that there was a nitrogen leak during that test. Both of those lines have now been updated.