The Artemis-I Moon rocket is going to be pulled from the launch pad in Florida because of a storm.
The SLS vehicle will be rolled back into its engineering workshop to be protected.
Ian is predicted to make a landfall in Florida on Thursday.
The Kennedy Space Center is expected to get a lot of rain.
Although the spaceport will probably escape the worst of the storm's impacts, it's not safe to have a multi-billion dollar rocket damaged.
The maiden flight of the SLS is likely to be moved to November due to the return of the Vehicle Assembly Building.
It was hoped that the storm's track through the Gulf would allow the rocket to stay on the pad so that it could be launched sooner.
The forecast models have seen the expected track shift eastward and put the west coast of Florida in the firing line.
There is a state of emergency in Florida.
The giant tractor is ready to roll back at the pad.
The journey to the VAB takes about half a day because of the slow speed. As soon as possible, engineers will want to get it up and running.
The retreat is expected to start at 0 400BST.
Humans will return to the moon after 50 years in a series of missions.
The first flight of the SLS is uncrewed and will send a capsule to the moon before returning to the ocean.
The next SLS-Orion outing is scheduled to take place in 2024. The loop around the Moon will be done again.
There is a chance that astronauts will go to the moon in the late 20th century.