CAPE CANAVERAL, Fla. – A pair of spacecraft rocketed toward the moon Saturday on the first mission dedicated to measuring lunar gravity and determining what’s inside Earth’s orbiting companion — all the way down to the core.

NASA launched the near-identical probes — named Grail-A and Grail-B — aboard a relatively small Delta II rocket to save money. It will take close to four months for the spacecraft to reach the moon, a long, roundabout journey compared with the three-day trip of the Apollo astronauts four decades ago.

Grail-A popped off the upper stage of the rocket exactly as planned 1½ hours after liftoff, followed eight minutes later by Grail-B. Both releases were seen live on NASA TV thanks to an on-board rocket camera, and generated loud applause in Launch Control.

The spacecraft are traveling independently to the moon, with A arriving on New Year’s Eve and B on New Year’s Day. They will conduct their science survey from a polar lunar orbit.

Beginning in March, once the spacecraft are orbiting just 34 miles above the moon’s surface, scientists will monitor the slight variations in distance between the two to map the moon’s entire gravitational field. The measurements will continue through May.

Once they were safely on their way, lead scientist Maria Zuber announced a contest for schoolchildren to replace the “working-class names” of Grail-A and Grail-B. She will help pick the winning names for the Grail twins later this year, well before the spacecraft reach the moon.

Zuber said she has her own pet names, “but I think I’ll keep those to myself because I don’t want to influence the contestants.” Some of the names used by members of her team over the four-year life of the project: Fred and Ginger, Castor and Pollux, and Tom and Jerry.