Euclid was built to map distant galaxies. A comet wandered into the picture anyway, and its refusal to sit still gave it away.
On October 19, 2026, the European Space Agency announced Euclid’s first discovery of a previously unknown comet: C/2026 S1 (Euclid). The telescope recorded it between September 16 and 24. The Minor Planet Center issued its confirmation on October 8. This is a newly announced discovery, not a comet first photographed today.
The blur was the clue
In the Euclid Consortium’s account of the search, the useful giveaway is wonderfully simple. Hold a telescope steady on background stars and galaxies, leave the camera gathering light, and a nearby moving object draws a short line across the image. A blur that looks like a nuisance can become the thing worth investigating.
University of Helsinki researcher Mikko Pöntinen developed software to pick out those moving streaks. His team checked the object’s position in other observations and compared it with known objects. The faint smudge also had a surrounding cloud, or coma, rather than looking like an ordinary point of light.
The distinction between spotting something and discovering something matters here. Euclid has seen known comets before. This one did not match the catalogued objects checked by the team. Ground-based observations then helped establish what the spacecraft had found.
The actual image is better than an artist’s impression
ESA’s September 20 image shows the evidence directly: a crowded black-and-white field on the left, with the comet enlarged on the right. The exposure lasted 560.52 seconds, long enough for the moving comet to stretch into a bright little dash inside its fuzzy cloud.
The inset is especially useful because the surrounding field contains plenty of other faint, fuzzy objects. Without the marked enlargement, the newcomer would be easy to overlook. This is not a close-up spacecraft visit or a dramatic view from a comet’s surface. It is a distant telescope observation with the interesting detail pulled out for inspection.
The Minor Planet Center’s October 8 circular supplies another check on the discovery. It lists four reports of cometary activity, including reports of a coma and one of a tail, alongside the observations used to characterize the object. That trail of measurements is what turns an intriguing streak into a recorded astronomical discovery.
A side quest with a useful payoff
Euclid’s main job is much larger in scale. Its galaxy survey is designed to investigate the Universe’s expansion and the influence of dark matter and dark energy. For that work, moving foreground objects have to be recognized so they do not confuse the view of the distant cosmos.
ESA says the discovery process also offers practice for finding potential targets for the future Comet Interceptor mission. There is an important catch: this particular comet is not a suitable target. Finding it demonstrates a useful search-and-alert process; it does not announce a new spacecraft destination.
That is the satisfying twist. The telescope was studying the deep background, but paying attention to something passing through the foreground added a new comet to the map.
Featured image: Comet C/2026 S1 (Euclid) observations in context. Credit: ESA/Euclid/Euclid Consortium/NASA, M. Pöntinen et al. 2026. Original image used unchanged under CC BY-SA 3.0 IGO. The source’s annotations and enlarged inset are retained.
