Nightfall on Space Launch Complex 2 at Vandenberg Air Force Base in California follows the rollback of the mobile service tower for launch of NASA's Orbiting Carbon Observatory-2, or OCO-2, aboard a United Launch Alliance Delta II rocket. Launch is scheduled for 5:56 a.m. EDT on July 1. OCO-2 is NASA’s first mission dedicated to studying atmospheric carbon dioxide, the leading human-produced greenhouse gas driving changes in Earth’s climate. OCO-2 will provide a new tool for understanding the human and natural sources of carbon dioxide emissions and the natural "sinks" that absorb carbon dioxide and help control its buildup. The observatory will measure the global geographic distribution of these sources and sinks and study their changes over time.