Europe's Euclid Telescope Finds a Comet It Was Built to Avoid

Keypoints:

• The European Space Agency (ESA) announced on 19 October that Euclid discovered a previously unknown comet
• The comet is named C/2026 S1 (Euclid) and appeared in images from 16 to 24 September 2026
• Euclid captured it in 16 exposures across five sub-fields of its Deep Field North survey
• The telescope deliberately avoids the asteroid- and comet-rich plane of the solar system
• The find is a test case for coordinating with ESA's future Comet Interceptor mission

ESA's Euclid space telescope has discovered its first new comet. The object is designated C/2026 S1 (Euclid). It turned up in images taken between 16 and 24 September 2026 during Euclid's Deep Field North survey, which covers a patch of sky in the constellation Draco. In the longest exposure, which lasted about 560 seconds, the comet appeared as a streak because it moved across the sky.

The discovery is a surprise. Euclid was built to map billions of galaxies and study dark matter and dark energy. To keep its deep images clean, it deliberately avoids the plane of the solar system, where dust and most asteroids and comets are concentrated. Dust there would add unwanted light and make faint galaxies harder to see.

The telescope's imaging style makes a comet find even less likely. Euclid uses a step-and-stare strategy and normally visits each patch of sky only once. Nothing can be re-imaged on demand to confirm what a moving object is. Euclid caught the comet in 16 exposures over five sub-fields. Follow-up inspection showed a fuzzy cloud around it, called a coma, which marks it as an active comet shedding dust and gas.

Archive scientist Bruno Altieri called it a happy coincidence. The comet's orbit and Euclid's scanning schedule overlapped in just the right way. Coverage of the ESA release places the comet about 1.3 billion kilometres from Earth, between the orbits of Jupiter and Saturn.

Speed mattered. The fast turnaround from identification to validation shows Euclid can spot new comets while they are still far from the Sun. That could help ESA's future Comet Interceptor mission, which is designed to meet a pristine comet. Euclid project scientist Valeria Pettorino said this comet is not a suitable target for that mission. She added that the discovery and alert process gives both teams a real test case for future coordination.