ICESat-2 (Ice, Cloud and land Elevation Satellite-2) is NASA’s active Earth-observation program launched on 15 September 2018. Its photon-counting Advanced Topographic Laser Altimeter System measures the elevation of ice sheets, glaciers, sea ice, vegetation canopies, inland and ocean surfaces in dense three-dimensional detail, enabling precise study of cryosphere change, sea-level rise, forests, and a changing Earth.