NASA hasn't seen this much prelaunch jitters since the space shuttle program ended last summer.
On Saturday, a private company is set to make history by launching a supply ship to the International Space Station. The rocket maker known as SpaceX hopes to join a short list of governments in flying to the orbiting lab.
NASA officials said Friday this is a "seminal moment" for spaceflight and an extremely important mission.
The SpaceX Falcon 9 rocket is scheduled to blast off from Cape Canaveral at precisely 4:55 a.m. Forecasters put the odds of good weather at 70 percent.
The Dragon capsule atop the rocket contains a half-ton of space station supplies. The capsule will perform practice maneuvers around the space station before NASA gives a "go" for docking on Tuesday.