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, its Instrument Design, and related Publications.

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.


  • 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 .
  • The COSI team gave presentations at the 37th International Cosmic Ray Conference (ICRC) in July. Proceedings papers have been completed and are available. See Tomsick et al., 2021 for the COSI-SMEX proceedings and Beechert et al., 2021 for a paper on the COSI (balloon) Al26 results.
  • The virtual site visit occurred in June as anticipated. Although virtual for external participants, the SSL-based COSI team was able to be in-person in the conference room. After some practice, the hybrid virtual/in-person visit worked well.
  • In March 2021, the COSI team submitted a Concept Study Report (see cover here) detailing the concept for COSI as a satellite. The team is now preparing for a virtual site visit from NASA in June 2021.
  • In August 2019, we proposed COSI to the NASA Small Explorer (SMEX) call.  On March 16th, 2020, it was announced that COSI was selected for a Phase A study!  The NASA press release is here.   Work on the Phase A study is underway!
  • The COSI group has two recent publications in ApJ on measurements of 511 keV emission:  Carolyn Kierans’ paper was accepted (Kierans et al., 2020, ApJ, 895, 44), and Thomas Siegert completed a paper on 511 keV imaging (Siegert et al., 2020, ApJ, 897, 45).
  • While the COSI balloon campaign was cancelled for 2020 due to travel restriction related to COVID-19, we are hoping to fly in 2021.  More details about the 2020 balloon campaign can found at COSI 2020.