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.


  • Members of the COSI team attended the 38th International Cosmic Ray Conference (ICRC) in July/August 2023, and conference proceedings papers are available:
  • The first COSI data challenge is publicly available and can be found here: https://github.com/cositools/cosi-data-challenge-1 . We would like to encourage anyone who is interested in learning more about COSI and COSI data analysis to participate!
  • The COSI Systems Requirements Review/Mission Definition Review (SRR/MDR) occurred at UC Berkeley’s Space Sciences Laboratory on January 24-25, 2023. The team did a great job presenting to the Standing Review Board (SRB), NASA representatives, and other attendees.
  • The COSI team was well-represented at the AAS meeting in Seattle, WA from January 8-12, 2023. Presenters included
    • Hannah Gulick on the Background and Transient Observer (BTO)
    • John Tomsick’s COSI update
    • Sean Pike’s talk on COSI and Galactic sources
    • Steve Boggs, Eric Burns, Carolyn Kierans, and Marco Ajello in a session on MeV Gamma-ray Astronomy in the 2020s and Beyond
    • Carolyn Kierans on uncovering the origin of Galactic positrons
    • Alyson Joens on studying GRBs with COSI
    • Marco Ajello on studying AGN with COSI
  • Older (but still important) news: COSI SMEX has been selected by NASA to continue its development as a small explorer mission: https://www.nasa.gov/press-release/nasa-selects-gamma-ray-telescope-to-chart-milky-way-evolution .