NASA Probes to Reach Moon Over New Year's Twin 'grails' will study moon's gravity field By Dustin Lushing, Newser Staff Posted Dec 28, 2011 5:57 PM CST 45 comments Comments This undated artist rendering provided by NASA shows the twin Grail spacecraft mapping the lunar gravity field. (AP Photo/NASA, Jet Propulsion Laboratory) (Newser) – Two NASA probes are poised to reach the moon over the New Year's holiday. After a journey of more than three months, the $496 million Grail probes—short for Gravity Recovery And Interior Laboratory—will soon be in place, reports AP. Grail A is set to arrive on New Year's Eve and Grail B the following day. The twin probes will not land on the moon's surface but rather fly 34 miles above it. One of the quandaries scientists hope to answer is why the moon's far side is more hilly than its Earthward side. "We actually know more about Mars ... than we do about our own moon," says the mission's chief scientist.