A pair of new spacecraft that will observe our planet’s ever-changing water cycle, ice sheets and crust is in final preparations for a California launch no earlier than Saturday, May 19. The Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) mission, a partnership between NASA and the German Research Centre for Geosciences (GFZ), will take over where the first GRACE mission left off when it completed its 15-year mission in 2017.