The laser instrument that launched into orbit last month aboard NASA’s Ice, Cloud and land Elevation Satellite-2 (ICESat-2) fired for the first time Sept. 30. With each of its 10,000 pulses per second, the instrument is sending 300 trillion green photons of light to the ground and measuring the travel time of the few that return: the method behind ICESat-2’s mission to monitor Earth’s changing ice. By the morning of Oct. 3, the satellite returned its first height measurements across the Antarctic ice sheet.