The Orion spacecraft hatch has been secured while teams troubleshoot.
If all goes well, astronauts Reid Wiseman, Victor Glover, Christina Koch, and Jeremy Hansen are set to blast off in a two-hour window starting at 6:24pm EDT.
Their journey will take them thousands of miles beyond the moon before looping back to Earth in under 10 days: no moon landing this time, but still a major step for future lunar missions.