The European Space Agency (ESA) reported this Monday, October 19, that its Euclid space telescope discovered its first previously unknown comet in images taken between September 16 and 24, 2026. For the ESA, Euclid is its dark universe detective, tasked with the difficult job of investigating how dark matter and dark energy have shaped the universe as it is known today.
Amid this complicated work, designed to take deep and stable images of distant galaxies, not to track fast-moving Solar System objects such as comets, Euclid had already detected several of these celestial bodies, but they were already known. However, last month it found C/2026 S1, the first new comet that Euclid has discovered, which was therefore also named after the telescope: Euclid.
The rapid response time between identification and validation demonstrated that the Euclid telescope can detect new comets while they are still relatively far from the Sun, which could make them suitable targets for the ESA’s future Comet Interceptor mission.
A Faint “Streak” in Several Images
Comets are icy and dusty remnants from the birth of the Solar System about 4.6 billion years ago, originating mainly in the distant Kuiper Belt and Oort Cloud, beyond the orbits of the outer planets of the Solar System (interstellar comets come from other solar systems).
Mikko Pontinen, from the University of Helsinki, together with the Euclid Consortium’s Solar System Objects Science Working Group, made the initial discovery using moving-object detection software that Mikko himself developed to monitor small Solar System objects in Euclid images.
“I was excited to see my software flagging a diffuse ‘streak’ in several images, the result of an object moving rapidly,” said this doctoral student, quoted in an ESA statement. “By predicting its position, I was able to identify it in other images, finding it in a total of five image sets. A subsequent inspection revealed that it had a diffuse cloud around it, indicative of an active object such as a comet shedding dust and gas.”
The comet was discovered in the “Euclid Deep Field North.” It was captured in five different fields of view over several days, with between two and four exposures per view, resulting in a total of 16 exposures.
Checks against catalogs of known comets and asteroids yielded no matching results, so Mikko reported the observations to the Minor Planet Center (MPC) of the International Astronomical Union as a new comet. This alerted ground-based astronomers, who were able to conduct follow-up observations. The MPC subsequently confirmed that it was a comet on October 8, and it is now identified as C/2026 S1 (Euclid).
Euclid, a Very Elusive Comet
The comet is located about 1.3 billion kilometers from Earth, a distance equivalent to that between the Sun and the orbit of Jupiter and Saturn, the ESA explains in its statement. However, it is not situated on the same horizontal plane as the planets; instead, it is currently pointing directly upward from the Sun’s perspective, tracing an arc over the Solar System in an orbit that will take thousands of years to complete.
Euclid is elusive and deliberately avoids the asteroid- and comet-rich band of the sky along the plane of the Solar System to reduce unwanted brightness from dust, which would make it difficult to obtain sensitive deep-space images. This makes the discovery of new Solar System objects particularly difficult.
Furthermore, the telescope’s “step-by-step” imaging strategy generally visits each section of the sky only once, so a comet cannot be observed again on demand to confirm its characteristics.
Nevertheless, tracking Solar System objects is important for correctly identifying all objects in the field of view and removing any “noise,” such as trails from moving comets or additional light, from Euclid images that is not relevant to its primary mission of precision galaxy mapping.
The goal of the Euclid mission is to reveal the hidden influence of dark matter and dark energy on the visible Universe. Over a period of six years, Euclid will observe the shapes and movements of billions of galaxies up to 10 billion light-years away.
“Euclid is expected to discover very few comets, so it was a happy coincidence that the combination of the comet’s orbit and Euclid’s scanning during those days captured the object in multiple images,” said Bruno Altieri, Euclid archive scientist and member of the discovery team, quoted in the same document.
“Although this particular comet is not a suitable candidate for the Comet Interceptor, the discovery and alert process gives us a real test of how our two mission teams will coordinate in the future,” added Valeria Pettorino, Euclid project scientist. “And, of course, we are pleased to have discovered a previously unknown comet so that the Solar System community can follow it up.”