A model of one of the twin Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) spacecraft is seen following a pre-launch briefing on the mission, Monday, April 30, 2018 at NASA Headquarters in Washington. The twin GRACE-FO spacecraft will measure and monitor monthly changes in how mass is redistributed within and among Earth's atmosphere, oceans, land and ice sheets, as well as within Earth itself. Photo Credit: (NASA/Joel Kowsky)