The European Space Agency’s Euclid space telescope accidentally discovered a distant, previously unknown comet designated C/2026 S1 while surveying faraway galaxies in September 2026, marking the first time the mission has spotted a previously unknown comet.
Space telescopes designed to peer billions of years into the past occasionally stumble upon unexpected neighbors. While mapping the large-scale structure of the cosmos to study dark matter and dark energy, the European Space Agency mission captured an interloper that had slipped past every existing sky survey.
#Euclid has discovered its first previously unknown comet. ☄️C/2026 S1 appeared in September images about 1.3 billion km from Earth, complete with a faint coma of dust and gas.A telescope built to study the dark universe just found a new neighbour.#Space #SpaceExploration pic.twitter.com/Wo8ZrU3O99
Mikko Pöntinen and Object-Detection Software Catch a Streaking Comet
The discovery began when Mikko Pöntinen of the University of Helsinki deployed custom object-detection software across data gathered during the spacecraft’s observations in the Euclid Deep Field North region of the telescope’s scans. Pöntinen worked alongside the Solar System Objects Science Working Group of the Euclid Consortium to identify small solar system objects in the telescope’s pictures.
I was excited to see my software flag a ‘streak’ smeared across several images — the result of a fast-moving object.
Mikko Pöntinen, University of Helsinki
Because the spacecraft relies on long exposures to capture faint, distant galaxies, a fast-moving solar system object appears as a smear rather than a sharp point of light. Pöntinen checked the flagged trajectory against known celestial catalogs and found no match, prompting a report to the International Astronomical Union’s Minor Planet Center. Ground-based follow-up observations helped confirm the find, and the Minor Planet Center officially designated the object as C/2026 S1 (Euclid) on October 8, 2026.
An Unusual Orbit High Above the Solar System
Most asteroids and comets travel within the flat, crowded plane where the planets circle the sun. The Euclid Space Telescope intentionally looks away from that dusty band to prevent glare from obscuring its deep-sky portraits of ancient galaxies. That deliberate positioning makes accidental solar system discoveries exceptionally rare.
Comet C/2026 S1 (Euclid) does not follow the standard flat trajectory. Positioned roughly 800 million miles (1.3 billion kilometers) from Earth—a distance comparable to a spot between the orbits of Jupiter and Saturn—the comet loops high above the solar system. Astronomers estimate that completing a single orbit takes thousands of years.
Bruno Altieri and Valeria Pettorino on a Happy Coincidence
Project scientists emphasized that the finding relied entirely on fortuitous timing rather than targeted searching. The comet’s sweeping path intersected the narrow window of space and time scanned by the observatory during its deep-field survey.
Very few comets are expected to be discovered by Euclid so it was a happy coincidence that the combination of the comet’s orbit and Euclid’s scanning over those days captured the object in multiple images.
Bruno Altieri, Euclid Archive Scientist
While the newly found icy body is not suitable as a target for upcoming dedicated missions, researchers noted that the accidental detection provides a valuable operational test.
While this particular comet is not a suitable candidate for Comet Interceptor, the process of discovery and alert gives us a real test case for how our two mission teams will coordinate in the future. And we are of course happy to have discovered a previously unknown comet for the solar system community to follow-up on.
Valeria Pettorino, Euclid project scientist
The Minor Planet Center confirmation makes C/2026 S1 (Euclid) one of 13 comets officially discovered during the year, adding a fresh object for ground-based telescopes to track as it continues its multi-thousand-year journey through the outer solar system.