  The Compton Spectrometer and Imager (COSI) is a NASA Small Explorer (SMEX)
satellite mission in development with a planned launch in 2027. COSI is a
wide-field gamma-ray telescope designed to survey the entire sky at 0.2-5 MeV.
It provides imaging, spectroscopy, and polarimetry of astrophysical sources,
and its germanium detectors provide excellent energy resolution for emission
line measurements. Science goals for COSI include studies of 0.511 MeV emission
from antimatter annihilation in the Galaxy, mapping radioactive elements from
nucleosynthesis, determining emission mechanisms and source geometries with
polarization measurements, and detecting and localizing multimessenger sources.
The instantaneous field of view for the germanium detectors is >25% of the sky,
and they are surrounded on the sides and bottom by active shields, providing
background rejection as well as allowing for detection of gamma-ray bursts and
other gamma-ray flares over most of the sky. In the following, we provide an
overview of the COSI mission, including the science, the technical design, and
the project status.
