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An artist's concept of GRAIL-B performing its lunar orbit insertion burn is seen in this undated NASA handout released to Reuters January 2, 2012. Two robotic probes began orbiting the moon Sunday in preparation for an unprecedented mission to map the lunar interior. NASA's twin Gravity Recovery And Interior Laboratory, or GRAIL, spacecraft wrapped up 2.6-million-mile journey to put themselves into lunar orbit on Saturday and Sunday. Over the next two months, the probes' 34-mile-high orbits will be adjusted to get them into optimal position to measure the pushes and pulls of the moon's gravity, data that scientists can use to model what is inside the moon. REUTERS/NASA/JPL-Caltech/Handout (UNITED STATES - Tags: SCIENCE TECHNOLOGY) THIS IMAGE HAS BEEN SUPPLIED BY A THIRD PARTY. IT IS DISTRIBUTED, EXACTLY AS RECEIVED BY REUTERS, AS A SERVICE TO CLIENTS. FOR EDITORIAL USE ONLY. NOT FOR SALE FOR MARKETING OR ADVERTISING CAMPAIGNS