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The Compton Spectrometer and Imager (COSI) is a soft gamma-ray survey telescope (0.2-5 MeV) designed to probe the origins of Galactic positrons, uncover the sites of nucleosynthesis in the Galaxy, perform pioneering studies of gamma-ray polarization, and find counterparts to multi-messenger sources. COSI’s compact Compton telescope combines improvement in sensitivity, spectral resolution, angular resolution, and sky coverage to facilitate groundbreaking science.  Learn more by reading about the Science COSI will explore and its Instrument Design.

The COSI mission has matured over decades through technology development with scientific balloon flights. Most notably, the COSI Team launched a balloon-borne version of the COSI instrument from Wanaka, New Zealand, in May 2016 aboard NASA’s Superpressure Balloon.  Learn about the balloon development work in the Balloon Blog.

The COSI Small Explorer NASA satellite is currently planned for launch in 2027.

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