A satellite image shows the Sun with a black dot at its centre, resembling a glaring eye in the darkness of space. The black spot is not a camera failure, but an expected behaviour of imaging sensors under extreme illumination, known as the Black Sun effect – when the over-saturation of pixels causes very bright areas to appear dark.
Taken by a miniature camera flying onboard the Trisat-S nanosatellite, this image is exactly what the mission team was hoping for.
“The Black Sun effect might at first appear to be an unwanted imaging artefact, but for us it is a very useful visual feature,” explains Iztok Kramberger, principal investigator of the Trisat programme at the University of Maribor, Slovenia. “Together with the observations of Earth and the Moon, the ‘Black Sun’ serves as a visual reference that can be used by a spacecraft to determine its attitude and orbit.”
The team already investigated this effect with their Trisat-R mission, which was supported by the European Space Agency’s General Support Technology Programme. When Trisat-R’s camera wasn’t busy capturing images of the Sun, it snapped an image of the Earth as seen through the camera’s 2-mm lens, from thousands of kilometres away.
Launched in early September with a follow-up experiment, Trisat-S is a mission led by the University of Maribor in cooperation with Slovenian space company SkyLabs and carries six miniature cameras that cover the satellite’s surroundings in all directions.
“While Trisat-R provides observations from two cameras in medium Earth orbit, offering a different perspective of the Earth and the space environment, Trisat-S gives us the opportunity to collect simultaneous observations from six cameras in low Earth orbit,” adds Iztok.
“The resulting images are particularly valuable because we can compare them with our ground-based simulations to further refine and validate our visual awareness models, closing the loop between simulations, laboratory testing and actual spaceflight.”
Earlier this year, the tiny imaging technology was exposed to an intense radiation environment in CERN’s Charm facility – similar to the harsh conditions it would experience in space – to further evaluate how radiation protection mechanisms developed by SkyLabs ensure that the camera sensor maintains its functionality and imaging performance under radiation.
Alongside its imaging-based navigation experiments, Trisat-S is demonstrating advanced spacecraft hibernation technologies for future long-duration missions.
“Ultimately, our goal is to give a spacecraft the ability to understand where it is and how it is oriented simply by looking at its surroundings using highly miniaturised technology, which is an important step towards autonomous navigation far beyond Earth,” concludes Iztok.
Trisat-S was one of six commercial and educational CubeSats launched on Isar Aerospace’s Spectrum launcher in September 2026. This launch made Isar Aerospace the first European company to reach orbit with their own launch vehicle and the first company to complete ESA’s European Launcher Challenge.