On July 8, 2026, UC Berkeley engineers and managers watched as a box was slowly lifted off a table. It is not a dramatic image, but it captures a real step for a spacecraft that has been years in the making. The box is the detector assembly for COSI, the Compton Spectrometer and Imager, and NASA published the photograph on September 18, 2026, under the title "COSI Telescope Comes Together". The image is credited to UC Berkeley/Alan Toth.
COSI is an Astrophysics Small Explorer, a wide-field gamma-ray telescope planned for launch in 2027. It is meant to study energetic phenomena in the Milky Way and beyond, including the creation and destruction of matter and antimatter and the final stages of stellar lives, according to NASA's description. Here is what the photo shows, where the mission stands, and what it is trying to solve.
What is actually in the photo
NASA's caption points to a small detail. On top of the detector box sit four pieces of silver material. They cover the flex circuits, which carry signals from the detectors to the readout electronics. It is a fitting part of the hardware to feature in an image marking assembly progress.
The caption does not say that the full observatory is finished, and we are not claiming it is. The image documents one stage of assembly, the raising of the detector assembly, on one date. What comes next is the work of integrating and testing the flight hardware ahead of a launch that NASA's mission page and the mission team still list as 2027.
A mission built by a collaboration
NASA describes COSI as a collaboration between UC Berkeley's Space Sciences Laboratory, UC San Diego, the Naval Research Laboratory, NASA Goddard, Northrop Grumman, the Space Dynamics Laboratory and the Italian Space Agency, along with a number of research institutions. The NASA Science mission page still labels COSI a future mission, with a 2027 launch and gamma rays as its wavelength.
The satellite has a predecessor. The mission page names the COSI balloon payload, and says the latest balloon flight was in 2016 aboard a NASA super pressure balloon. A related item on the same page says that balloon, carrying an early version of COSI, launched from Wanaka Airport, New Zealand, on May 17, 2016. The satellite design, the page says, is the result of technology development tested on scientific balloons over the past two decades.
The road so far
The UC Berkeley mission site keeps a project timeline that shows how the hardware got here:
- April 16-17, 2024: KDP-C confirmation and the start of Phase C.
- July 2024: NASA selected SpaceX to launch COSI, planned for 2027 on a Falcon 9 from Cape Canaveral.
- December 4-6, 2024: Critical Design Review at Northrop Grumman in Dulles, which the team reports as successful.
- May 2025: The engineering-model cryostat, with 16 germanium and dummy detectors and two shield walls, passed a cold vacuum test.
- July 2025: The cryocooler and heat-removal subsystem began thermal vacuum testing at Goddard.
- October 28, 2025: Flight-model cryostat assembly began, with the first germanium detector installed.
- November 20, 2025: Sixteen detectors were installed.
One caveat for readers who go looking. The Berkeley site's newest dated entry in its updates list is from November 2025, so it does not cover the July 2026 detector-assembly milestone. The NASA image article is the source for that event.
Where it will fly, and what it will see
COSI is designed to survey the sky from 0.2 to 5 MeV. NASA's mission page notes that this is up to millions of times the energy of visible light. A preflight study by Gallego and colleagues, arXiv:2510.25304, describes COSI as a Compton telescope built around a compact array of cross-strip germanium detectors. The paper, accepted by The Astrophysical Journal on December 30, 2025, gives the planned orbit as equatorial and low-Earth, at 530 kilometres, with a prime mission of two years.
That paper is about a less glamorous problem than antimatter: background. The authors write that observation of MeV gamma rays is dominated by background, mostly from extragalactic and atmospheric photons but also from activation of the detector materials by cosmic-ray interactions. They ran Monte Carlo simulations covering the first three months in orbit and extrapolated them to two years. Their result is that extragalactic photons dominate the background below 660 keV, while delayed activation from cosmic-ray primaries and albedo photons dominate at higher energies.
They also report a discrepancy in a widely used radiation model. As part of the work, the authors compared recent South Atlantic Anomaly measurements at low latitude (below 1 degree) from HEPD-01 on the CSES-01 satellite with the AP9/AE9 model, and found that the model overestimates the flux by a factor of 9.
The four questions COSI is built to answer
NASA lists four science goals.
Where do the galaxy's positrons come from? Electrons and positrons annihilate in a flash of gamma rays when they meet. Astronomers have observed this emission, at about 0.5 MeV (511 keV), from near the Milky Way's centre since the 1970s, and NASA says its source has remained a mystery. It is the goal closest to the antimatter theme of the mission, and the line falls inside COSI's 0.2 to 5 MeV range.
Where are elements made? NASA says that observing light produced by specific elements, and comparing light from different isotopes, reveals information about the evolution of massive stars and the supernova explosions that end their lives. It adds that these measurements are only accessible in the gamma-ray range COSI will detect, allowing scientists to see where new elements are forming in the galaxy.
What does gamma-ray polarization reveal? Polarization measurements provide information about how emission is produced and the shape of the source. NASA says COSI will measure the gamma-ray polarization of accreting black holes at the centres of galaxies for the first time, and will also study polarized light from smaller black holes, pulsars and distant gamma-ray bursts.
How can gamma rays help with multimessenger events? Multimessenger astronomy combines light with other signals, such as gravitational waves and neutrinos. NASA says COSI will detect and localize short gamma-ray bursts to allow rapid follow-up observations with other telescopes, and the mission is meant to find counterparts to sources detected by means other than light.
Why It Matters
The 0.5 MeV emission from near the galactic centre has been observed since the 1970s, and NASA still describes its source as a mystery. COSI is designed to probe the origins of the galaxy's antimatter, and that is the reason a photograph of a box being lifted deserves attention.
The mission also shows how much of spaceflight is slow, cumulative engineering. The timeline runs from a 2024 confirmation, through design review and engineering-model tests in 2025, to flight detectors going into the cryostat and now the detector assembly being raised in July 2026. The launch is still planned, not scheduled to the day. Nothing in the sources gives a specific launch date beyond the year 2027.
Finally, the mission's scope reaches past a single mystery: antimatter, element formation, polarization and multimessenger events. The prime mission is planned at two years in low-Earth orbit, and the preflight paper states that background simulation and identification are crucial for the data analysis, which is why that unglamorous work matters to what the data can eventually deliver.
