NASA's latest moonshot was delayed again until Saturday so engineers could make sure the rocket was safe to fly.
High wind halted the countdown for the first launch attempt Thursday, and NASA aimed for Friday. Then late Thursday night, they bumped the launch to Saturday.
Space agency officials said they needed time to analyze data from the core engine of the unmanned Delta II rocket. As the rocket was drained of propellants Thursday, an engine heater apparently stayed on longer than usual. Engineers wanted to make sure no repairs were needed.
Everything checked out fine, and the rocket was cleared for flight, said NASA spokesman Allard Beutel.
Saturday's liftoff time is 8:29 a.m. Forecasters put the odds of favorable weather at 60 percent; the outlook improves Sunday.
The near identical twin satellites are named Grail-A and Grail-B. Grail stands for Gravity Recovery and Interior Laboratory.
Scientists will measure the entire gravity field of the moon by keeping precise track of the distance between the two probes as they chase one another around Earth's closest neighbor. They hope to figure out what's beneath the moon's surface, all the way down to the core.
None of the world's previous 109 moon missions has been dedicated to lunar gravity like Grail.
The $496 million Grail mission also is reaching out to schoolchildren. Each spacecraft has four cameras; middle school children can order up snapshots of the moon during the science operation phase of the project, which will run from March through May.
America's first spacewoman, Sally Ride, is leading the MoonKAM educational effort.
Sally Ride Science: https://www.sallyridescience.com/home