Published: Mar 07, 2012 3:04 PM EST

LOS ANGELES (AP) - Twin NASA probes in orbit around the moon have begun mapping the lunar gravity field in hopes of figuring out why Earth's only natural satellite is shaped the way it is. The campaign kicked off late Tuesday, two months after the spacecraft arrived back-to-back at the moon over the New Year's weekend. Despite numerous missions to the moon, scientists still don't know why the far side of the moon is more mountainous than the side that always faces Earth. The $496 million mission is officially called the Gravity Recovery And Interior Laboratory. It was launched last September from Florida. Probes entered orbit around the moon in January but couldn't start collecting data right away because they had to tweak their positions until they were 35 miles above the surface.