An unmanned US rocket was launched on Saturday from Cape Canaveral in Florida to deliver twin robotic probes to the moon in the hope of learning what is inside.
Less than two hours later, both probes were flying freely from the rocket's upper-stage motor and were communicating with NASA.
Liftoff of the Delta 2 rocket occurred two days later than planned after an earlier attempt was scrubbed on Thursday.
The twin satellites on board are headed to a point in space 1.5 million km away where gravitational pull from the Sun and Earth balances out.
From there, the Gravity Recovery and Interior Laboratory satellites will make a long, slow approach to the moon, arriving on 31 December and 1 January.
The twin probes are designed to precisely map the moon's gravity so scientists can learn what lies beneath the lunar crust and whether the moon's core is solid, liquid or some combination of the two.
The small probes are designed to fly single file over the lunar poles, mapping the dips and swells in lunar gravity.
Linked by radio waves, the spacecraft will be able to detect changes in the tug of lunar gravity as small as one micron.
The measurements are so precise that scientists have to factor out a number of other forces, including the pressure of sunlight and the gravitational influences of all other planets in the solar system, even Pluto, which is about 4.7 billion km away.
Scientists believe the moon's building blocks were large chunks of debris jettisoned from Earth after a collision with an object as big as Mars.
