File photo of a Delta 4 rocket's countdown rehearsal at launch pad 37B. Credit: Boeing

A countdown simulation for the upcoming Delta 4 rocket launch carrying a next-generation weather satellite has uncovered a liquid oxygen leak on the booster's first stage, a problem that will delay liftoff at least a couple of weeks.

Officials announced Friday that the launch would slip from April 28 to mid-May as a result.

Standing atop pad 37B at Cape Canaveral Air Force Station, the Delta 4 underwent a full launch day rehearsal Wednesday. The mobile service tower was rolled back and the two-stage rocket was filled with supercold liquid hydrogen and liquid oxygen while the launch team practiced the countdown procedures.

But the test, which is a standard exercise in the lead up to launch, revealed the liquid oxygen leak on the first stage. A NASA statement says a fill and drain valve could be the source of the leak.

The liquid hydrogen loading on the first stage was successfully completed, as well as filling of the upper stage with its cryogenic propellants.

Engineers are developing a plan to isolate and repair the leak, and another fueling test will be performed to ensure the problem has been resolved before resuming launch preparations, NASA said.

The earliest possible launch date, based on the work ahead of the team, would be May 12, the space agency's statement said. However, that's currently the planned day for shuttle Atlantis' liftoff on the Hubble Space Telescope servicing mission.

"Launch is targeted to occur no earlier than May 12. The date will be adjusted accordingly based on the outcome of troubleshooting and readiness of the Delta 4, the status of space shuttle Atlantis on STS-125 and the availability of the Eastern Range," NASA said.

The GOES O weather satellite remains at the Astrotech complex in Titusville. On Tuesday, the craft was encapsulated in the two-piece fairing that serves as the rocket's nose cone.

The satellite's transfer to pad 37B for mounting atop the Delta 4 will be delayed until after the upcoming fueling test. NASA projects the earliest that the satellite could make its way to the pad is April 24.

The Geostationary Operational Environmental Satellite O is the second in an updated series of weather observatories with sharper vision and extended life. More details about the launch can be found in our earlier story.