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, a pair of spacecraft the size of washing machines are set to enter orbit around it in the latest lunar mission. Their job is to measure the uneven gravity field and determine what lies beneath — straight down to the core.
Since rocketing from the Florida coast in September, the near-identical Grail spacecraft have been independently traveling to their destination and will arrive 24 hours apart.
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 could be captured into orbit. This move will be repeated by the other the following day.
"I know I'm going to be nervous. I'm definitely a worrywart," said project manager David Lehman of NASA's Jet Propulsion Laboratory, which oversees the $496 million, three-month mission.
Once in orbit, the spacecraft will spend the next two months flying in formation and chasing each other 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.
Scientists expect the mission to yield a bounty of new information about the moon, but don't count on the United States sending astronauts back any time soon. President Barack Obama favors landing on an asteroid as a stepping stone to Mars.