LOS ANGELES — 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 can be captured into orbit. This move will be repeated by the other the following day.
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.
