The lunar twins of NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission have each completed a rocket burn that has sealed their fate. The burns modified the orbit of the formation-flying spacecraft. Over the next three days, this new orbit will carry the twins lower and lower over the moon’s surface. On Monday afternoon, Dec. 17, at about 2:28 p.m. PST (5:28 p.m. EST), their moon-skimming will conclude when a portion of the lunar surface — an unnamed mountain near the natural satellite’s north pole — rises higher than their orbital altitude.