NASA's Artemis 1 moon rocket will launch from Florida.
Launch window opens at 1.04am local time (6.04am UK) - but late repairs have delayed specific launch time.
Earlier, NASA had to perform a late repair on a potentially dangerous leak. Another issue has also emerged - and that's a lost radar signal considered critical to the launch.
A faulty ethernet switch, which aids communication between ground teams at Kennedy Space Center and elsewhere, was identified as the cause. It's now been replaced, but needs to be tested.
The long-awaited launch comes after several postponements and marks the first step in NASA's bid to put humans back on the moon.
Orion spacecraft is being carried by a Space Launch System rocket - the most powerful vehicle ever blasted into space.
No humans aboard the Orion this time, but mission will pave the way for crewed Artemis 2 and 3 missions in the next few years.
News ID : 1524