The moon has come a long way since Galileo first peered at it through a telescope. Unmanned probes have circled around it and landed on its surface. Twelve American astronauts have walked on it. And lunar rocks and soil have been hauled back from it.
Despite being well-studied, Earth's closest neighbor remains an enigma.
Over the New Year's weekend, two spacecraft the size of washing machines are set to enter orbit around it in the latest lunar mission. Their job is to measure the moon's uneven gravity field and determine what lies beneath -- straight down to the core.
Since rocketing from the Florida coast in September, the Grail spacecraft have been traveling to their destination and will arrive 24 hours apart. Their paths are right on target.
"Both spacecraft have performed essentially flawlessly since launch, but one can never take anything for granted in this business," said mission chief scientist Maria Zuber of Massachusetts Institute of Technology.
The nail-biting part is yet to come. On New Year's Eve, one of the Grail probes -- short for Gravity Recovery and Interior Laboratory -- will fire its engine to slow down so that it can be captured into lunar orbit. This move will be repeated by the other the following day.
Engineers said the chances of the probes overshooting are slim since their trajectories have been precise.
But getting struck by a cosmic ray could prevent the completion of the engine burn, meaning they wouldn't get boosted into the right orbit.
Once in orbit, the spacecraft will spend the next two months flying in formation and chasing one another around the moon until they are about 35 miles above the surface with an average separation of 124 miles. Data collection won't begin until March.
As the probes circle the moon, regional changes in the lunar gravity field will cause them to speed up or slow down. This in turn will change the distance between them. Radio signals transmitted by the spacecraft will measure the slight distance gaps, allowing researchers to map the underlying gravity field.
Using the gravity information, scientists can deduce what's below or at the lunar surface such as mountains and craters. That may help explain why the far side of the moon is more rugged than the side that faces Earth.
The probes are officially known as Grail-A and Grail-B. Several months ago, NASA hosted a contest inviting schools and students to submit new names. The probes will be christened with the winning names after the second probe goes into orbit, Zuber said.