Euclid Was Built to Map Galaxies. Its Archive Also Caught a Comet.

 Image: [“A composite of eight Euclid VIS exposures of Comet C/2026 S1 (Euclid)”](https://www.esa.int/ESA_Multimedia/Images/2026/10/A_composite_of_eight_Euclid_VIS_exposures_of_Comet_C_2026_S1_Euclid) — ESA/Euclid/Euclid Consortium/NASA, M. Pöntinen et al. 2026; [CC BY-SA 3.0 IGO](https://creativecommons.org/licenses/by-sa/3.0/igo/), unchanged. ESA’s Euclid telescope was designed to make deep, steady images of distant galaxies. In September 2026, moving-object software developed by University of Helsinki researcher Mikko Pöntinen picked out something much closer: the first previously unknown comet discovered by Euclid, now designated C/2026 S1 (Euclid). The telescope had imaged known comets before, but this one was new to the catalogues. The software flagged a streak in Euclid Deep Field North. Pöntinen predicted where the object should appear and found it in five image sets: 16 exposures taken between September 16 and 24. Combining shorter exposures along its predicted path revealed a fuzzy coma of dust and gas. After checks against known comets and asteroids found no match, the team reported it to the International Astronomical Union’s Minor Planet Center, which confirmed it on October 8. This was an unusual catch. Euclid avoids the dusty plane where many small Solar System bodies are found, and its “step-and-stare” survey usually moves on after imaging a patch of sky. Yet moving objects still matter to its galaxy maps: their streaks can add unwanted light, so identifying them helps researchers separate foreground objects from distant galaxies. This comet is about 1.3 billion kilometres from Earth and follows an orbit that takes thousands of years; more observations will refine the estimate. C/2026 S1 is not a target for ESA’s future Comet Interceptor mission. Its value is as a real-world test of how Euclid’s archive and moving-object alerts can help find and validate rare visitors hidden in survey data. [ESA’s discovery report](https://www.esa.int/Science_Exploration/Space_Science/Euclid/Euclid_discovers_new_comet) explains the process. ~science