NASA's uncrewed Artemis I launch of the Space Launch System rocket and Orion capsule was called off on Monday.

Why it matters: NASA plans to use the SLS rocket and Orion to return people to the Moon for the first time since the end of the Apollo program in the 1970s.

The latest: An engine issue prevented the SLS from getting off the pad at Kennedy Space Center in Florida.

• One of the rocket's four engines didn't get up to the correct temperature, preventing the space agency from launching.
• NASA has backup launch dates on Sept. 2 and Sept. 5, but it wasn't immediately clear whether the space agency would be able to fix the issues and target one of those days.

How it works: For this launch, the SLS is expected to send the Orion capsule on a journey around the Moon before it comes back to Earth, splashing down under parachutes in the ocean.

• The test is expected to prove out all of the capabilities needed to one day send people to the Moon.
• NASA's first crewed Artemis Moon landing is currently expected to occur in 2025.