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.


  • COSI has been confirmed and is proceeding to Phase C! The Key Decision Point (KDP) review occurred on April 16, 2024, and the KDP-C Decision Memo was signed on April 16-17. This marks the end of the mission formulation phase and the start of implementation.
  • The second COSI data challenge is publicly available and can be found here: https://github.com/cositools/cosi-data-challenge-2 . This is a great opportunity to try out the COSI software and provide feedback to the team.
  • The COSI Preliminary Design Review (PDR) occurred at Northrop Grumman (Dulles) on February 27-29, 2024. Congratulations to the team for an excellent review.
  • 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.
  • 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 .