With a launch pad gas leak fixed, NASA will try to launch a Delta II rocket and two missile defense demonstration satellites at 8 a.m. Friday.
The latest Air Force weather forecast offers a 70-percent chance of conditions good enough for liftoff during the one-hour window, with cumulus clouds and showers again the main concern.
Heavy clouds moving in from the Atlantic Ocean scrubbed a first launch attempt Wednesday.
A second attempt was planned this morning, but a leak from fuel line at Cape Canaveral Air Force Station's Launch Complex 17B prompted another delay.
Engineers traced the leak to a corroded weld, located beneath the launch pad, in the line through which nearly 10,000 gallons of RP-1 rocket fuel will be pumped from a storage tank to the rocket.
The weld was repaired quickly enough to limit the delay to one day, and NASA says the rocket and spacecraft are ready to fly.
Managers gave the "go" for a Friday attempt during a meeting at 9 a.m. this morning.
NASA's Launch Services Program is managing the mission for the Missile Defense Agency, with United Launch Alliance providing the Delta II launch vehicle.
Atop the 127-foot rocket are two satellites that will test visible and infrared cameras designed to detect and track ballistic missiles, and to provide information accurate enough to help missile defense assets shoot them down.
Watch the launch preparations live on NASA TV.