First comet for the detective of the dark universe

The Euclid mission of the European Space Agency (ESA) has discovered its first comet. It is a long-period comet that takes over three hundred thousand years to complete an orbit around the Sun, is located about 1.3 billion kilometers from Earth, and has been named C/2026 S1 (Euclid). Euclid, which scans the sky to make a census of billions of galaxies, captured the small celestial body in a series of sixteen images between September 16 and 24, 2026, while observing the Euclid Deep Field North. This is one of the three deep fields, small pieces of sky that the mission is scrutinizing in great detail, in addition to the broader but less deep investigation being carried out over a third of the celestial sphere. The signal left by the comet was revealed by Mikko Pöntinen, a doctoral student at the University of Helsinki, who is part of the working group dedicated to objects in the Solar System within the Euclid collaboration. "I was excited to see my software signal a 'streak' spread across several images, the result of a fast-moving object," comments Pöntinen. "By predicting its position, I managed to identify it in other images, finding it in a total of five series of images. Further inspection revealed that it was surrounded by a blurred cloud, indicative of an active celestial body, like a comet, which releases dust and gas." After checking catalogs of known comets and asteroids without success, the discovery was reported to the Minor Planet Center (MPC), the organization tasked by the International Astronomical Union of collecting and safeguarding data on small bodies in the Solar System, which issued an alert to observers around the world. Several telescopes in Switzerland, Austria, Hungary, Romania, Canada, the United States, and Hawaii observed the celestial body between the end of September and the beginning of October, allowing confirmation of its nature as a comet, announced with a circular on October 8th. The comet, which is currently between the orbits of Jupiter and Saturn, does not move along the plane on which the planets of the Solar System lie but moves around the Sun along an elliptical orbit, very elongated, perpendicular to this plane. It has a magnitude of 20.7 and is not visible to the naked eye. Even though Euclid was designed to capture deep images of distant galaxies, not fast-moving celestial bodies like asteroids or comets, it has already observed some known comets in the past. This is the first discovery of a comet ever observed before, a remarkable result for a space mission with a completely different objective – the study of dark matter and dark energy. Moreover, Euclid is certainly not indifferent to the celestial bodies of the Solar System, to which it dedicates particular attention to correctly identifying all objects captured in the images and removing light from non-cosmological sources. However, it is rather rare for it to discover new objects in our cosmic neighborhood, as the telescope deliberately avoids the region most populated by asteroids and comets to minimize the glare of dust in the deep sky images, and it usually does not revisit the same parts of the sky, so it is difficult for it to pick up an object moving between fields – as happened in this case during the observations of the Euclid Deep Field North. "It is expected that Euclid will discover very few comets, so it was a nice coincidence that the combination of the comet's orbit and the scan performed by Euclid on those days allowed for immortalizing the object in multiple images," states Bruno Altieri, an ESA Euclid archive scientist and part of the team that made the discovery. The speed with which the discovery was identified and verified also indicates that Euclid is capable of discovering new comets when they are still relatively far from the Sun, making it a possible ally for the future ESA Comet Interceptor mission, which will launch at the end of this decade with a comet that has not yet been discovered. "Although this particular comet is not a suitable candidate for Comet Interceptor, the discovery and alert process offer us a real test for the future coordination of our two mission teams," adds Valeria Pettorino, an ESA Euclid project scientist. "Of course, we are very happy to have discovered a previously unknown comet that the scientific community studying the Solar System can further study." To learn more: - Read about the circular from the Minor Planet Center on the website: Granvik, M.; Altieri, B.; Carry, B.; Pontinen, M.; Irureta-Goyena, B. Y.; Nucita, A. A.; Euclid; Kruk, S.; Franco, A.; Ye, Q.-Z.; Collaboration, Z. T. F.; Sárneczky, K.; Spratt, C. E.; Balam, D. D.; Green, D. W. E.; Prosperi, E.; Jaeger, M.; Rhemann, G.; Lachat, D.; Maikner, J.; Sato, H.; Ursache, F.; Hudin, L.; Flewelling, H.; Weryk, R.; Wainscoat, R.; Acohido, A. Correction: the magnitude of the comet is 20.7 and not 10.66 as erroneously indicated in a previous version.