Euclid discovers new comet

The discovery [1] is impressive for a mission that is designed to take deep, steady images of distant galaxies – not for following fast-moving Solar System objects like comets [2]. Furthermore, the quick turnaround between identifying and validation proves the case that Euclid can detect new comets while they are still relatively far from the Sun, potentially leading to a suitable target for ESA’s future Comet Interceptor mission.

Mikko Pöntinen of the University of Helsinki [3], together with the Solar System Objects Science Working Group of the Euclid Consortium, made the initial discovery using moving object detection software that Mikko developed to monitor small Solar System objects in Euclid’s images: “I was excited to see my software flag a ‘streak’ smeared across several images – the result of a fast-moving object. By predicting its position I could identify it in other images, in total finding it in five image sets [4]. Further inspection revealed that it had a fuzzy cloud around it, indicative of an active object like a comet that is shedding dust and gas.”

Checks against catalogues of known comets and asteroids found no match, so Mikko reported the observations to the International Astronomical Union’s Minor Planet Center (MPC) as a new comet [5]. This alerted ground-based astronomers who could then contribute follow-up observations. The MPC subsequently confirmed it as a comet on 8 October, and it is now identified as C/2026 S1 (Euclid).

The comet is located approximately 1.3 billion kilometres away from Earth, equivalent to the distance from the Sun to roughly between the orbits of Jupiter and Saturn. However, it does not lie in the same horizontal plane as the planets, but is currently ‘straight up’ from the Sun’s perspective, arching over the Solar System on an orbit that will take thousands of years to complete.

Euclid deliberately avoids the asteroid- and comet-rich band of sky along the Solar System’s plane to reduce unwanted glare from dust that would hinder sensitive deep-sky imaging, making the discovery of new Solar System objects particularly challenging. In addition, the telescope’s ‘step-and-stare’ imaging strategy typically only visits each section of the sky once, so a comet cannot simply be revisited on demand to confirm its characteristics.

Nonetheless, tracking Solar System objects is important in order to correctly identify all objects in the field of view, and to remove any ‘noise’ – like moving comet trails or extra light – from Euclid’s images that is not relevant to its primary mission of precision galaxy mapping [6].

“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,” says Bruno Altieri, Euclid Archive Scientist, and part of the discovery team. “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,” adds Valeria Pettorino, Euclid Project Scientist.

Background information

[1] Euclid has previously detected comets, but they were already known. C/2026 S1 (Euclid) is the first new comet that Euclid has discovered.

[2] Comets are icy, dusty leftovers from the birth of the Solar System about 4.6 billion years ago, mostly originating in the distant Kuiper Belt and Oort Cloud, beyond the orbits of the outer Solar System planets (interstellar comets come from other solar systems). When a comet swings close to the Sun, its ice turns to gas, creating a glowing fuzzy ‘coma’ and long tail. Because comets originating in our Solar System have changed very little since they formed, studying them helps scientists understand the raw ingredients and conditions that shaped the planets, including clues about where Earth’s water and the building blocks of life may have come from.

[3] Mikko Pöntinen is a PhD student at the University of Helsinki. The discovery was reported on behalf of the Solar System Objects Science Working Group of the Euclid Consortium: M. Pöntinen, A. A. Nucita, A. Franco, M. Granvik, B. Altieri, S. Kruk, B. Y. Irureta-Goyena and B. Carry.

[4] 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, yielding a total of 16 exposures.

[5] The International Astronomical Union’s Minor Planet Center (MPC) acts as the global clearing house for comet and asteroid observations, collecting reports from observers worldwide and using them to confirm new discoveries, calculate orbits, and assign official designations. The orbital period of C/2026 S1 (Euclid) will be refined once more data has been collected.

[6] 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 motions of billions of galaxies out to 10 billion light-years.

About Euclid

Euclid is a European mission, built and operated by ESA, with contributions from its Member States and NASA. The Euclid Consortium – consisting of more than 2000 scientists from 300 institutes in 15 European countries, the USA, Canada and Japan – is responsible for providing the scientific instruments and scientific data analysis. ESA selected Thales Alenia Space as prime contractor for the construction of the satellite and its service module, with Airbus Defence and Space chosen to develop the payload module, including the telescope. NASA provided the detectors of the Near-Infrared Spectrometer and Photometer, NISP. Euclid is a medium-class mission in ESA’s Cosmic Vision Programme.

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