The Compton Spectrometer and Imager (COSI), a gamma-ray telescope, has completed its assembly phase at the University of California, Berkeley. This milestone brings the mission, scheduled for launch in 2027, closer to its goal of mapping the sky for low-energy gamma-ray sources and studying extreme astrophysical phenomena.

COSI is designed to detect and measure the polarization of gamma rays in the 0.2 to 5 MeV range. This capability will allow scientists to investigate the origin of positrons in our galaxy, study the explosive nucleosynthesis of heavy elements in supernovae and kilonovae, and explore particle acceleration mechanisms in compact objects such as pulsars and black holes. Gamma-ray polarization information is crucial for unraveling the geometry and magnetic fields of these cosmic sources.

The assembly of the main detector, which includes four pieces of silver material covering the flex circuits for signal transmission, represents a critical step. Following this phase, COSI will undergo rigorous environmental and calibration tests to ensure its proper functioning in the extreme conditions of space. The COSI mission, selected by NASA in 2021 as part of its Small Explorer (SMEX) program, promises to open a new window into understanding the high-energy universe.