Today, NASA rolled out the completed core section of its massive new rocket, the Space Launch System, which is designed to take people into deep space. The core stage, manufactured at NASA’s Michoud Assembly Facility in New Orleans, Louisiana, is now heading to Mississippi where it will undergo key testing before it can launch for the first time.

The Space Launch System, or SLS, is a critical part of NASA’s Artemis program, an initiative to return humans to the surface of the Moon by 2024. When it’s complete, it’ll be the most powerful rocket in the world, rivaling that of the Saturn V rocket that took the first astronauts to the Moon. However, the rocket has yet to actually fly. The SLS has been in development for most of the last decade, experiencing multiple delays and rising costs. Originally, the monster rocket was meant to debut as early as 2017; now, it probably won’t launch until 2021 at the earliest.

The SLS has been in development for most of the last decade

Now, the core will travel by boat to NASA’s Stennis Space Center in Mississippi. There, the stage will undergo a major test meant to run through all the biggest steps of launch without actually sending the rocket to space. Called the Green Run Test, the event will entail filling the core up with propellant and igniting stage’s four main engines, just as they would during flight. However, the rocket will be enclosed in a test stand, not on a launchpad. This hot fire will last for about eight minutes, which is the length of time the main engines should burn during an actual launch.

NASA and Boeing, the primary contractor of the SLS, expect the Green Run Test to occur sometime this summer, and argue the rocket could then ship out to Kennedy Space Center in Cape Canaveral, Florida, anywhere between July and October. But there’s still a lot to do once the rocket makes it to the Cape. The upper portion of the rocket will be stacked on top of the core, then NASA’s Orion crew capsule will also be lofted on top of the rocket. For the first SLS flight, no people will actually be inside the Orion, but it will test out the capsule’s performance in space.

NASA still doesn’t have a solid launch date for this first flight, dubbed Artemis 1. But once it launches, the agency hopes to fly another SLS again just a year later, this time with crew on board. Boeing argues that the company has learned lessons from building this first core stage and says that building the next SLS is going 40 percent faster, according to John Shannon, vice president and program manager for the SLS at Boeing. He noted that welding turned out to be more complicated than expected and that building the engine section of the stage was a challenge, as well. “We expected to have the rocket built really by the end of ‘17, so we’re about two years late,” Shannon said on a press call. “Boeing completely owns that.”

Speedy development of the next rockets will be key if NASA hopes to meet its deadline of sending humans to the Moon within the next four years. In the meantime, NASA still needs to develop numerous other elements to get humans to the lunar surface, such as space suits and lunar modules. This year will be a formative one for the agency, revealing if it can actually get all of these vehicles ready in time to safely transport humans to Earth’s neighbor.