Aura is NASA’s active Earth-observation atmospheric-chemistry program, launched on 15 July 2004 to measure ozone, aerosols, and trace gases throughout the atmosphere. Its Microwave Limb Sounder, Ozone Monitoring Instrument, Tropospheric Emission Spectrometer, and High Resolution Dynamics Limb Sounder provide global observations of atmospheric composition, air quality, ozone trends, and chemistry–climate interactions from the A-Train orbit.