A pair of NASA gravity probes designed to look deep beneath the lunar surface are poised to lift off Sept. 8  from Florida’s Cape Canaveral Air Force Station onboard a United Launch Alliance Delta 2 rocket. On Aug. 31, mission managers finished up the flight readiness review for the twin GRAIL spacecraft, a major milestone on the path to liftoff.

If the unmanned spacecraft launch on time and all goes well, the pair will arrive at lunar orbit around New Year’s Day, and then begin studying the structure of Earth’s nearest neighbor in unprecedented detail. The probes’ observations should help scientists get a better handle on how the Moon formed and evolved, researchers said.

“GRAIL will unlock lunar mysteries and help us understand how the Moon, Earth and other rocky planets evolved as well,” said GRAIL principal investigator Maria Zuber of the Massachusetts Institute of Technology in a statement.

The two GRAIL — short for Gravity Recovery and Interior Laboratory —spacecraft will take nearly four months to reach the Moon.The GRAIL A probe should settle into lunar orbit on Dec. 31, with its twin GRAIL B arriving a day later.

The $496 million spacecraft are taking a long, looping trip via the sun-Earth Lagrange Point 1, a gravitationally stable spot between our planet and the sun. This route is energy-efficient, researchers said, helping keep the mission’s costs down.

GRAIL has two one-second launch windows on Sept. 8; it can lift off at either 8:37 a.m. or 9:16 a.m. EDT. The launch period extends until Oct. 19.