In the early morning of 15 November 2005, the unmanned Altair aircraft returned to Gray Butte Airfield, north of Los Angeles, Calif., after completing an 18.4?hour mission over the eastern Pacific Ocean. The flight was the last in a series undertaken by the U.S. National Oceanic and Atmospheric Administration (NOAA) in its Unmanned Aircraft System (UAS) Demonstration Project. The successful flight series has helped start the era of unmanned flights in service of environmental goals. Altair cruised at altitudes in the lower stratosphere (13 kilometers; ˜43,000 feet), collecting atmospheric data with a 140?kilogram payload of both remote and in situ instruments. NOAA has recognized that UAS technology will improve its ability to meet scientific and operational objectives in the coming years. Operating sensor payloads on a UAS fleet could play a crucial role in the detection and attribution of climate change, improvement of weather predictions, management of water resources, monitoring and evaluation of ecosystems and sanctuaries, and atmospheric and oceanic research. UAS platforms have the potential to carry instrument payloads to remote locations in a manner that could not otherwise be achieved with conventionally piloted aircraft.