The U.S. space agency, NASA, has launched its twin GRAIL lunar satellites meant to examine the gravitational field and internal structure of the moon.

The launch of the Gravity Recovery and Interior Laboratory, or GRAIL, followed several weather-related delays Thursday and Friday. Saturday's launch took place in the second and final of two “launch windows” that NASA had announced for the day.

High winds scrubbed Thursday's first launch effort. NASA officials say they rescheduled the Friday attempt when technicians asked for more time to analyze the condition of the Delta II launch rocket.

The GRAIL mission is to create the first complete map of the moon's gravitational field and reveal new details of its internal structure, down to the lunar core. Scientists hope to learn more about how the moon was formed, and whether its rugged mountain ranges might have resulted from a collision with a smaller second moon, around four billion years ago.

The two GRAIL spacecraft will orbit the moon for 90 days. About six weeks after their work is done, NASA said the pair will plunge into the lunar surface. Scientific analysis of their data is expected to continue for a year.