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.


  • Many members of the team enjoyed the in-person(!) HEAD meeting in Pittsburgh during the week of March 14-17, 2022. There were two COSI talks: one focusing on polarization measurements by Hadar Lazar and one general COSI talk by John Tomsick. Also, there were the following posters:
    • Beechert et al., “Charge Sharing in Cross-Strip Germanium Detectors”
    • Gulick et al., “Detector System for Time-Domain Astrophysics in the Soft Gamma-ray Regime” (Winner of the Student Poster Award!)
    • Beechert et al., “Measurement of emission lines by COSI”
    • Karwin et al., “The COSI Data Challenges and Simulations”
    • Roberts et al., “The Performance Characteristics of the NRL2 ASIC Designed for COSI”
    • Roberts et al., “Analysis Pipeline Tools and Discrete Imaging Capabilities for COSI”
  • Although we were not able to fly the COSI balloon payload in 2020, we took a lot of calibration data, and a paper on calibration methods including data from the 2016 and 2020 campaigns has been accepted in Nuclear Instruments and Methods in Physics Research Section A. The paper is entitled, “Calibrations of the Compton Spectrometer and Imager” by Beechert, Lazar, et al. (2022), and a preprint is available here: arXiv:2203.00695.
  • Results continue to be obtained from the data taken during the 2016 COSI balloon flight. A paper entitled, “Measurement of Galactic Al26 with the Compton Spectrometer and Imager” by Beechert et al. (2022) was accepted by the Astrophysical Journal, and the paper is available here.
  • 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.