The Earth Observing System (EOS) Aura satellite mission is scheduled to launch in June 2004. The Aura mission will make significantly improved measurements of atmospheric constituents. Aura is designed to tackle three science questionsc Is the ozone layer recovering as expected? What are the sources and processes that control tropospheric pollutants? And, what is the quantitative impact of constituents on climate change? Aura will answer these questions by globally measuring a comprehensive set of trace gases and aerosols (Table 1) at high vertical and horizontal resolution. Figure 1 shows the Aura spacecraft and its four instruments.