Two-day demonstration, the last major milestone before launch to the moon, will be concluded Sunday when teams load almost 1 million gallons super-cold fuel onto the rocket. The 9-second mark will be the last time engines start to ignite.

NASA will set a launch date once it has analyzed the results from the Space Launch System dress rehearsal — SLS, for short.

Officials believe the rocket could launch as soon as June and send the Orion crew capsule, attached to it, hurtling towards the moon. The capsule will be in space for at least one month before returning to Earth.

The first moonshot since the Apollo lunar landings half-century ago will not see anyone onboard. The second test flight, which will loop around the moon and return, is scheduled for 2024. It will be piloted by astronauts. NASA says this would allow astronauts to land on the moon in 2025.

However, the U.S. Government Accountability Office warned that technical problems remain, primarily with the lunar landing lander and spacesuits. This could cause further delays to the already delayed moon landing. GAO also mentioned billions of dollars in increasing costs.

The rocket, which measures 322 feet (90.8 meters) in height, made its debut at Kennedy Space Center’s launch pad two weeks ago. All of the rocket’s systems have been turned on in preparation for the weekend’s test. Officials warned that the rehearsal could be delayed by technical issues or thunderstorms.

NASA promised updates over the weekend but the public will not be able listen. Security concerns were cited by the space agency.

“We are being cautious, an abundance of caution — especially in the environment that we find ourselves in today,” stated Tom Whitmeyer (head of NASA’s exploration system development).

NASA plans to announce the crews of the first lunar missions this summer. Nine men and nine women are part of the pool. Two are already at the International Space Station, while two more are expected to arrive in the next few weeks.

From 1968 to 1972, 24 astronauts flew to Apollo; 12 landed on lunar soil.

NASA, unlike Apollo, is working with private businesses to launch its moon program. It’s called Artemis, after Apollo’s twin sister from Greek mythology. SpaceX’s Starship, still in development, will transport astronauts to lunar orbit. NASA’s capsule and rocket will launch them into lunar orbit. NASA is looking for additional companies to assist with future landings.

The goal of the space agency is to establish a permanent moon presence and then move on to Mars. NASA Administrator Bill Nelson mentioned 2040 as the goal for a Martian expedition, with astronauts.